At first, I thought that I must have been hitting some button or mistakenly spinning the Super Knob and causing the distortion but it now happens with such frequency and I'm paying close attention to the keyboard and know I'm not inadvertently hitting a button or the Super Knob.

That was my first thought.

I took a quick look at Two Acoustics 1 and the first 3 elements all use A.SW OFF for XA control.
Only one of the 3 will sound at a time because they are velocity switched - so depending on the velocity one, and only one, of the elements will sound.

But if either assign switch is turned on, even accidentally the sound will be quite different.

That isn't your issue if you know you aren't changing either of the assign switch switches.

Each of those 3 elements has slightly different Level and Level/Vel settings and radically different Curve settings so those will also affect the sound from one test to another if the velocity is different.

Element 1 velocity is 1-70, element 2 is 71- 109 and 3 is the rest. So if one test has a velocity below 71 and the next is 71 or higher you will get different sounds.

Elements 1 and 2 have radically different filter cutoff settings so that will make each elements sound different depending on the velocity.

Not suggesting the above is definitive for your use case just mentioning those things since they are the most common way sound changes occur unexpectedly in the preset performances.

1. Which grand piano performance are you using?
2. Can you reproduce the problem 'on demand'? If so can you provide the exact setup for that?

The sound kind of became a synthetic distortion of the original sound, sort of like a Chorus that was electronically humming along with some reverb and hollowness and a synthesizer added to the sound.

This part of your description "sounds" like a phase issue to me. Improper MIDI routing can cause this. What's your MIDI setup? Including the 5-pin DIN connections and USB connections.

Andrew, I'm using Full Concert Grand for the piano sound. Jason, I don't use a MIDI connection.

After exhaustively following Andrew's ideas, dove deep into the settings, and carefully monitored the keyboard when the sound shift randomly happens, I think I've figured it out. For a handful of my songs, I use the Mod Wheel. Since I use the Full Concert Grand and Two Acoustics 1 more than other performances--and the Mod Wheel should be at 0 for those two--I think what's happening is that if the Mod Wheel was not turned all the way down from the previous Performance and was left about half way on the dial, the vibration from rigorous playing very slightly shifts the Mod Wheel UP causing a distortion in the sound. The Wheel is not totally loose but it moves very freely. Not sure why this has suddenly started to happen because the MODX worked fine for about 18 months, but I can replicate the sound shift that is occurring by moving the Mod Wheel.

Now that I know what is happening, I think it can be fixed. I might just need to use some blue painters tape to hold the Wheel in whatever position I need it to be sure that it doesn't shift slightly causing the distortion. I played for several hours this past weekend and made sure the Mod Wheel was in the minimal position (0). The issue never came up unless I manually moved the Mod Wheel.

Thank you for your help.

For a handful of my songs, I use the Mod Wheel. Since I use the Full Concert Grand and Two Acoustics 1 more than other performances--and the Mod Wheel should be at 0 for those two--I think what's happening is that if the Mod Wheel was not turned all the way down from the previous Performance and was left about half way on the dial, the vibration from rigorous playing very slightly shifts the Mod Wheel UP causing a distortion in the sound.

Good work Id'ing the problem - I think I can tell you WHY that is happening.

By default Controllers will 'Reset' when you change performances. See page 192 of the Reference doc - https://usa.yamaha.com/products/music_production/synthesizers/modx/downloads.html

Hold/Reset (Controller Hold/Reset)
Determines the status of the controllers (Modulation Wheel, Aftertouch, Foot Controller, Breath Controller, Knobs, etc.) when switching between Performances. When this is set to “Hold,” the controllers are kept at the current setting. When this is set to “Reset,” the controllers are reset to the default states (below).
Settings: Hold, Reset

The DEFAULT setting is Reset if you haven't changed it at some point in your ownership.

As the table shows that means the Mod Wheel wll reset to Minimum (0) when you change performances.

BUT (notice the big BUT) - that reset of 0 is the LOGICAL position. The PHYSICAL position will be wherever you left it. If you left it in the middle the physical position is 64 but the logical position is 0.

Guess what happens if you, or vibration, nudge the mod wheel even a little bit. The effect of the mod wheel will first be 0 and then jump immediately to 64 when the nudge happens.

You can test that yourself:

1. select a 3rd performance
2. set the Mod wheel to the middle
3. select Full Concert Grand
4. move the mod wheel (it is still physically in the middle) just a tad upward
5. you should hear a jump in the sound

I checked both performances (Full Concert Grand and Two Acoustics 1) and they both have assignments for the Mod Wheel.

Since you need the RESET for those two performances the only 'solution' is the one you found: find a way to tighten up the mod wheel movement so vibration doesn't move it.

EXTRA CREDIT: The reason knobs don't manifest the same problem is because the knobs do NOT have a physical min/max position. So no matter how you twiddle them when they reset to logical 0 that is also their physical 0.

If I were in your shoes I would delete the modwheel assignment so it doesn't offset any parameter(s) and then, if necessary, compensate by adjusting the parameters directly.

Then the mod wheel won't do anything and your Performances sound like they should.

[STORE] your modified Performances to the User bank and use your own creations without modwheel assignments.

Andrew and Jason, thank you for the excellent suggestions. I'm traveling and won't be back home for another week. I'm anxious to get back into the unit and delete the Mod Wheel assignments for those 2 performances. That seems like the safest way to address it, especially since I use those 2 Performances more than any other. Andrew, I'll bet you are absolutely correct in your analysis. That makes sense. Will test this in a week when I return. Thanks again for all your help!