...this was one of the main reasons I upgraded from the moxf8 to not have the sound suddenly cut off.

Then when you get your MODX8, you will want to invest some time to learn the terminology and how that particular feature works.

We always discuss programming the Motion Control Engine from two perspectives... near the surface (snorkeler’s) view and the deeper (scuba diver’s) view, where you get creative...

Seamless transition between instrument sounds at the snorkeler’s level is called SSS. Seamless Sound Switching. This allows the user with no knowledge of the deeper regions of programming, to move from any four Part Performance displaying the “SSS” icon to any other Performance. While others here will point out that Arpeggios stop, but obviously, Arpeggios are not ‘sound’, but are notes/controller data generated by a separate device...of course you will notice they stop (but Arpeggios are not ‘sound’ any more than touching the keyboard is ‘sound’... it’s what triggers sound). If you expect Arps to transition between programs THEY DON’T. They are not sound.

That aside, any four Part Performance can be transitioned to any other four Part Performance, that are sounding instruments Parts, without any interruption in the sound of notes being held on by the keys/Sustain pedal. Typical: while holding strings you move to piano and begin playing, the strings will continue until you ‘release’ them.

Seamless transition between instrument sounds at the scuba diver’s level, has you creating a program by “merging” instrument sounds into a Performance so that you can “invent” the specific musical transition you require. Say you perform in church and need an Organ, Orchestral Strings, Orchestral Brass, and Choir sounds for a particular number. If you scuba dive, you would “merge” these four programs into a Performance under KBD CTRL (Keyboard Control). Because you can always transition seamlessly between sounds within the same Performance, you can design your own custom seamless transitions between these four sounds... using the many options within the Motion Control Synthesis Engine; designing different combinations, and/ or the ability to access each individually. The four selected sounds could come from other Performances... merged in a way that suits your specific requirement.

Typically, you need to transition from one sound to another. As you begin to program ‘your own’ you start to be freed from someone else’s idea about how it should transition, to a place where you can creatively invent what works exactly for your musical situation.

Sure you could place an individual organ, into a Live Set, and move seamlessly to the individual Orchestral Strings in the next Live Set slot, and then to the individual Orchestral Brass in the next Live Set slot, and finally to the Choir Sound in the next — as long as you meet the SSS requirements... but when you feel comfortable enough to create your own Multi Part Performance that integrates all four sounds in a way that directly supports what you’re doing musically... that’s when you reach a level of satisfaction that is unparalleled.

Mastering MODX: Performance Basics 1
Mastering MODX: Performance Basics 2 and the “Live Set”

I can't remember exactly but when I 1st got it, it seemed like I could switch between any performance without the sound suddenly cutting out; Now there are only certain patches in which I can do this. Its seems pretty arbitrary. I tried looking for certain characteristics in certain patches that maybe doesn't allow it to work but Im not finding any distinguishable differences in which one patch would work but others wouldn't.

As BM essentially pointed out, the "distinguishable difference" you're looking for is that it works as long as the Performances you're switching between each consist of sounds only in Parts 1 through 4. If you have more than 4 Parts (OR if one of your Parts is located in 5-16), it won't work. These Performances are also marked with a little purple SSS symbol, to let you know they are available for seamless sound switching.

Comparing to your MOXF8:
... In Voice mode, you could not seamlessly switch from one patch to another. On MODX, all the same Voices are available as Single Part Performances, but now you can seamlessly switch from one of these to another.
... In Performance mode, which permitted combinations of up-to-4 sounds, you could not seamlessly switch from one Performance to another. On MODX, you can seamlessly switch from one up-to-4 sound combination to another using its Performances. A difference here is that a MOXF Performance was limited to 4 parts. On the MODX, you have the option of creating Performances with more than 4 parts, but when you do, you will no longer have the seamless switching. But as long as you stay within the same 4-Part max that you had on the MOXF, you have the seamless switching.
...In Mix mode, you could create sets of up to 16 sounds and seamlessly switch among those sounds, singly or in groups. This is something I am unclear about myself, in terms of how this function translates to the MODX. As long as you're working with combinations of 1-4 sounds, you can duplicate that behavior. But IIRC, in the MOXF Mix mode, you could, for example, set up a mix that consisted of three sets of five sounds each, and seamlessly switch among those three combinations. I don't know if there is any way to replicate that behavior on a MODX, i.e. with some kind of "scuba diving" approach, as BM put it. It can't be done the same way as the MOXF did it, because that relied on being able to independently assign the different parts to different MIDI channels, which you can't do on the MODX, but maybe there's another way to do it...? Though it's also a kind of specific case that may not come up often, having to switch among sounds with more than 4 Parts within a bank of sounds that doesn't exceed 16 total.

...In Mix mode, you could create sets of up to 16 sounds and seamlessly switch among those sounds, singly or in groups. This is something I am unclear about myself, in terms of how this function translates to the MODX.

You can setup 16 sounds and seamlessly switch among them, but singly; as you correctly surmised, you cannot freely group them.

You get to create just one 'group', that being the KBD CTRL Parts 1-8. Remember, the difference in the MODX is that those Parts linked by KBD CTRL are not controlled in the same way that PERFORMANCE Parts or the MIXING Parts were in the MOXF. In PERFORMANCE mode on the MOXF all sounds were on the same MIDI channel, by default.

In the MOXF MIXING when you 'group', you are doing so by placing PARTS on the same MIDI Receive Channel. You have to do this because the MOXF only transmitted on one MIDI channel at a time.

All PARTs being on the same channel means they all respond to controllers in a like fashion, unless you opt-out (Rcv Sw = Off). Altogether now... FILTER OPEN!!! Altogether now, FILTER CLOSE!!!
In the MODX engine you can define control over each Filter within each PART directly (each AWM2 Part has as many as 8 Filters), You can program each to move differently, then with a single gesture command precise change, if you wish. The depth of control is entirely different and fantastically expanded.

Because MOXF in MIXING mode is transmitting from the Keyboard through the Sequencer Track to the Tone Generator. So you can place several Tone Generator PARTS on the same MIDI Channel to respond, in kind, to the data. A simple Channel layering.

In the MODX, the equivalent 16 Part multi-timbral mode, you can transmit from the Keyboard on as many as eight MIDI Channels, simultaneously. You have KNOB and Control matrix where a single gesture can send differing amounts to different items under your control.
_ Each of the 16 PARTS responds to the Super Knob;
_ Each of the 16 PARTS responds to SCENE change events;
_ Each of the 16 PARTS has 8 Assign Knobs, which can be linked, as you define, to the Super Knob's movement.
_ Each of the, up to 8, KBD CTRL Parts can all be addressed by NOTE-ONs from the Keyboard and the usual physical Controllers (PB, MW, FC, AsSw 1/2, etc), simultaneously.

Each individual PART is the equivalent of what a MOXF VOICE was... so when you can 'merge' 8 of these into a KBD CTRL PERFORMANCE, it can be a very power fun thing. If you build an 8 PART KBD CTRL Performance - PARTS 9-16 can be setup as eight individuals (in waiting). They are 'in waiting' because you must selected them, to play them... you can seamlessly switch from your 8 KBD CTRL entity and any of the Singles occupying 9 through 16. You can always seamlessly switch between PARTS with a PERFORMANCE - and a PERFORMANCE has 16 PARTS!

If you KBD CTRL Performance is only two PARTs then Part 3-16 are available to be setup as singles.
Remember, just because you place multiple PARTS under KBD CTRL, does not mean they are sounding at all times; the concept of building the sounds you want to transition to in among your KBD CTRL Parts is a key to 'getting' what the Motion Control Synthesis Engine is all about.