could be a failing cable or connection somewhere but since it's intermittent it's gonna be tricky to replicate/track down.
I'd try another ribbon cable first and volume pot if you can, visually inspect the caps for leakage/popped tops (it could be that a cap is failing but only after a certain point).
of course do the usual pull the chips and clean the legs, and check your voltages as well.
let us know how you make out.