Very strange happened one night.  Although the utility appeared normal, the keyboard became drastically out of tune after being on for about two hours.  Because of this, we ended up having to switch the keyboard.

Later we checked the tuning with a tuning device and it was correct.  I presume because it was turned off for at least 45 mins. 

Any thoughts on what is causing this?

Often this type of problem is caused by a poor contact in the pitch bend (PB) control.

Try tapping on the PB wheel, and move it side to side a bit (don’t rotate it). If the pitch changes that way, the control either needs cleaning or is worn/defective.

Another possibility is the PB wiring connection to the DMSUB pc board.

Pitch changes can also occur if voltage isn’t correct, but usually there are other symptoms if that’s the problem.

Thanks.  I will pass this info along.