MOTM

Could someone please help me with this problem? Please?
Assembled a 420, it was working great, right. Left it on for a while, 
suddenly the audible frequency when self oscillating has moved way to 
the clockwise part of the dial, as in nothing audible until about 7 or 
8. tried fussing with the trimpots to no avail. didn't think it's being 
on for several hours would cause such a problem but I can't seem to 
think of anything else. could anyone suggest fixes, i was enjoying 
using it too much to be without it now.
Paul