Discussion about the Korg PolySix synthesizer

Hello everyone;

I'm currently trying to fix two Polysix (including fittin a new processor
board) and both show similar behaviour. I am not sure if this is actually
normal (since the oscillators are true VCO's and not DCO's) or if there is
something wrong with those units.
All calibrations went very well and the unit is good in tune. However it
displays a very slight irregular "trembling" or "wavering" in the tuning -
sort of like a very small random modulation. It's a bit as if the pitch
jumps a little bit to the side and is immediately pulled back to the
regular pitch. When playing a chord or using the modulation effect it
becomes practically inaudible; the pitch deviation is perhaps 1 cent (when
it happens).
First I thought that this could come from some instability in the power
supply. However when making the filter self-oscillate the generated tone is
completely stable without the "trembling" phenomenom hence I don't think
this is a power supply issue.

Since it's been some time since I last worked on a Polysix: Is that perhaps
even normal and in the nature of the VCO's? Or is this a known problem?

Thanks,

Malte