I've had stability issues with 60s and 80s relating to heat.
If kept on, they're not too bad, but have had issues if they've been switched off for Ny length of time (a day or so) and back on.
I've also noticed that the stability mod on the VCOs is very often done badly.
The diode being slathered in heat sink compound and bent over the VCO chip... Usually sitting a mm or so above the actual chip, and not actually in contact.
I'd recommend using a heat transfer epoxy to bond the diode to the surface of the VCO. That way it's going to work much better.
If you need to replace the VCO chip, replace the diode as well.
I'd also recommend letting a unit warm up properly before getting too in depth in calibration and debugging. A CS80 at a clients studio took 2 weeks to stabilize after being switched on. As the days went by more and more areas started working properly, and have been pretty stable since. That said a few new faults crept in, but these were easily fixed by rechipping the aftertouch assignment and keyboard assignment sections.