Korg Poly800/EX800 Users

Hi, I've searched and haven't found anyone else posting with this problem.  Hopefully I'm not just posting in the wrong places.
My poly 800 drifts out of tune over time, often just over the span of a day, and it's usually too far to use the tuning slider on the face of the keyboard to correct it.  I have opened it up and tuned it using the trimpots VR1 and VR4, but the problem persists, and opening the keyboard up before each gig is a hassle.
Does anyone have experience with this issue?  I don't know enough to judge whether it's a problem with caps, heat, or some other type of shenanigan.
Thanks for reading!
Ian