I also have an XV-88 and have noticed some strange behavior - distortion overload type of sound - on certain patches when playing only a certain combination of notes simultaneously ( as in a chord ). Similar to what you described either power down the up or changing the patch and then coming back to it cleared it up.

Not sure if your synth is sick or not - or if mine is. I have a feeling there may be a few bugs in some of these synths.

I have a couple of question for you.

Have you done much messing around with aftertouch? It seems that the aftertouch on my XV-88 is unusually hard to trigger! I was wondering if you felt the same way about your XV-88.

Also whith velocity set to REAL, it seems as though I have to play exceptionally hard to get certain patches - like piano ones - to respond to the high velocity playing. On many of these patches the high velocity produces a diffrent timbre of sound. Do feel you have to play maybe a bit too hard on your XV-88 to get the velocity needed to trigger the change?

Let me know what you find out about your stuck note problem.