Vintage Synth Repair

hi everyone:
i've been playing a moog prodigy for close to 15 years now and have run into this problem 
for the first time:
when i'm playing the keyboard will sometimes drop or rise in pitch uncontrollably. it might 
drop a semitone or two and then stay there for a while, and then rise quickly and drop 
again... if i turn it off it often will right itself but i can't be doing that when i'm playing live, 
which i do often. 
has anybody experienced this before?
-J