Before concluding that there is something amiss, let’s consider the things that can cause this behavior. Because the pitch corrects itself by changing programs (a Reset command is sent), and because MIDI messages can cause this type of change, and MIDI values persist until you either reset them or a new value for the controller in question is sent....

We must, therefore, look for any device or item that might be connected to the keyboard (we don’t make anything called an “X90” but we’ll assume you have a Yamaha S90 XS (that’s as close as we get to an X90) since you posted in this forum.

If you are playing back a MIDI File, this can leave a PB value hanging if stopped before resetting to +0000... MIDI messages ‘persist’ - this means if you stop playback in the middle of a Pitch Bend event, the pitch will remain detuned until you reset it or send a different PB message.
An Arpeggio can contain pitch values.
Aftertouch can be applied to pitch. Some instruments use Aftertouch to bend the pitch (in a negative direction) allowing key pressure to dip the pitch (some harmonica sounds use this)... but pitch should return to normal as you ease up on key pressure.* (*this makes some sounds unplayable by the ‘heavy-handed’)
Something connected to the synth via MIDI or USB-MIDI can be sending a message affecting the tuning.
Assignable Function switches can be programmed to bring in different articulations - many of which adjust pitch.
The S90 XS can be a Remote Control Surface for DAW software via USB-MIDI... if you have incorrectly routed the Remote Control messages you maybe arbitrarily sending messages that are affecting tuning. Port 2 via USB should be used for Remote commands only.

Anyway, check what else sends MIDI messages — temporarily disconnect it. If the problem goes away, then take the time to troubleshoot what the cause is. Since digital instruments don’t generally drift out of tune at all on their own, (simply not how they break) we feel there is a definite cause... especially since changing programs fixes it.

Try wiggling the PB wheel to see if that also corrects it.
It does not have to be the PB Wheel, it could be Master Tuning, Coarse or Fine Tuning, etc.
Let us know ...start with...what else is connected to your synth?

Hello Bad Mister:

Thank you for looking into this. Some good news......

1. I call it the S90 for short but yes it is the S90XS.

2. There are no other devises or connections to the S90XS. I am a snorkeler at best and the Midi stuff was well over my head. Can't say I followed it but I am thinking it wasn't a Midi problem.

3. I did "wiggled" the PB and there was a very quiet snap. I wonder if the wheel had come off its axis or....??? I don't know the mechanics but wonder if it got bumped and dislodged. I don't take it out of the music room but you never know if one of those night people hit it and didn't tell me. Anyway I have taken it for quite a spin on a number of days and all seems to be OK I am very happy. Thank you for directing me to the PB.

I love my S90XS but unfortunately I am getting a bit of arthritis. I don't want to give in to a synth key bed yet but was wondering about the CP88. I would only swap the S90XS out for the action. Unfortunately there are not many places to do an A/B for me. Could you comment on the action for me with arthritis?

Thank you again Bad Mister. I enjoy your Youtube videos.

I love my S90XS but unfortunately I am getting a bit of arthritis. I don't want to give in to a synth key bed yet but was wondering about the CP88. I would only swap the S90XS out for the action. Unfortunately there are not many places to do an A/B for me. Could you comment on the action for me with arthritis?

Sorry, I really can’t make that decision for you. You are aware that only you know how your hands feel and whether or not the change from the S90 XS action to the CP88’s action will be an improvement or more of the same. You don’t need many places just one authorized a Yamaha Synth dealer. Music, and the joy of playing is worth the trip (imho). Call your dealer, arrange to visit if/when they have both in stock.