Vintage Synth Repair

I'm having a bit of a problem lately with my Moog Prodigy. It seems 
to be getting worse.. The frequencies of the two oscillators seem to 
be out of proportion with each other in regards to the keyboard. If I 
play a low A and tune osc 2, the high A will beat terribly. Then if I 
tune them at the high A, they'll be way out of tune on the low A. 
Everything seemed fine for the first 15 minutes or so last night, but 
then it started do this again (I've noticed it the passed couple 
practices). They're in tune if I sync them, but I don't always want 
that sound. Any ideas? I need to fix this fast!
Thanks
Mike