What you are describing sounds like it could be issues with the CD4051B's used to demux the DAC
for the sample and hold circuits. That would be Z101,102, 201,202, etc. There are two of these for each voice.
4051's from that era do fail, and are readily available and cheap.

Another possibility would be the TL084/LF347 quad op amps in the S&H circuits. Also available and cheap.
You could try swapping them between voice 6 and a "good" voice to see if the problem
moves. I think these chips would be in sockets in the Polaris, right?

I really doubt it's issues with polystyrene caps as they rarely fail,
unlike some other types. I would look elsewhere.

Bob