spencered, I am a victim of the same behaviour.
It happens with one oboe or one of the violins in a string setup, or with one of two clarinets. They will become out of tune by one half note even, just randomly. I normally stop playback (as it becomes unbearable) and restart playback. Sometimes I am lucky and those instruments start at the right pitch again, sometimes not. I can sort of work against it by turning the pitch bend wheel of my MIDI keyboard.
I promised myself to dismantle the keyboard and clean its inside of any dust. The two wheels seem to work optically, so dust might be the culprit: