Vintage Synth Repair

I have one of those 'white elephant' nicknamed Moog/Cordovox organ 
things. One note ('G' I think) in every octave likes to go out of 
tune, way out. I tried to pop this mother open and retune it. Fine. 
It lasted about 10 minutes, then was just as bad as before. How do I 
fix this?

Craig Storm