Discussion about the Korg PolySix synthesizer

Hello,

i recently got a Polysix. This is a great synth i really love. This unit had the leaking battery issue but has been repaired and works fine.
There is yet a problem. Sometimes the P6 can get out of tune but it is just for a moment or i could say like a "jump" in tuning. I can also hear some "plock" sounds sometimes. It is really strange as I used it last sunday during 4 hours and it worked perfectly. Yesterday, as i was playing with my band, it wasn't tuned, even with the tune knob at the minimum (so the unit was too high). I don't know if there is a relation with it, but when i plug a tuning stompboxe (with adaptor), P6 went to normal tuning, with tune knob at middle position (I did the tuning with service manual recently).
So is there a problem of Power supply (the "plock" are not normal i suppose). Does it come from the two filter capacitors?
Is the P6 sensible with the gear we plug with it?

Thanks for your help,

regards,

Ed.