You've got some old analog ones then, huh? I actually know a few things about those. Some of them you may find a tune knob / button - some not.
The tuning can vary with heat / coolness so flip that baby on and give it some time to warm up before playing live or in the studio.
A lot of times it can be old electronics too, but it's just a thing they do. You have Voltage Controlled Ocillators, probably so there isn't much you can do to make them stay perfectly in tune, except regulate / check / repair the power in the unit.
I've heard you can get a voltage regulator as well - but I don't know much about that. Also have heard that you can use power conditioners (like Furmans) to keep things as stable as possible on the incoming power, which some ppl say has helped them tremendously.
Other than that - you're getting into replacing capacitors and such.
I was an old Amplifier repairman so I had coworkers who dealt with vintage synths, but never really messed with them much myself. It seems like from what I remember since you have VCO's you need to focus on stabilizing voltage.