Have you done the tuning procedure on those voices lately? The OB-8 has the about the easiest tuning calibration of any analog synth. It just consists of turning the trimmers until 2 LEDs light up... See the service manual.
If those voices still fail try swapping the CEM3340s to other voices and see if the problem follows them. If you need any CEMs I have them in stock.
i've noticed that one pin of small tape from midi interface is broken. i've repaired it, also checked all connections from midi to cpu board. now after powering up it makes me sth like this. btw - when i disconnect voiceboard and keyboard it's still same
My OB-Xa did this because of a bad chip in the s&h circuit for the autotune circuit, I believe it was a tl084. I don't know how similar they are, but the circuit was on the bottom right of the bottom board for my later era OB-Xa.