tuning shifts flat after playing a few minutes

Somewhere in the back of my mind, I remember encountering a problem with my M-Audio Keystation Pro where a CC setting in Full-Featured mode was being used the by the DAW (I think for some internal signal processing) or the controller had a minor short (it was a lot years ago) that caused the tuning to change during playback and slowly go flat.  I can't remember what the CC controller or parameter was, but switching to the Minimalistic MIDI profile helped a lot, and I eventually was able to diagnose which CC or MIDI event was getting sent incorrectly or mishandled.

Try the Minimalistic MIDI profile or watch the recent MIDI event window and see if anything stands out as causing the tuning change.  You can also, in a preset, turn off MIDI events for particular sliders and settings, if you determine which is the only sending it flat.  Often, if settings in Pianoteq aren't visibly changing or you're in Stage, it makes it more likely that it's the pitch bend event getting activated by something.