Discussion about the Korg PolySix synthesizer

Hi, I have a Polysix that plays in tune at start-up, but after 30 or so minutes the "C", for example, is moved up the keyboard and plays where the "E" is. It does this consistently. Is this the problem with the OPTOCOUPLER? Is it really true that the Polysix needs to warm up fully before you can hear it's true tuning? I noticed that the back middle of the synth gets hot after ~30min. Is that normal? I've read some things, but am pretty new to the workings of the Polysix. Thanks for any insight. -Frank