How quickly does it go from one side to the other, couple of seconds or more like thirty?
First off inspect all the bits, reflow any dodgy joints, replace any bad parts. Then split it left right and check pre and power sections on their own. Odds are you'll find it by then.
Its meant to be dual mono though, so short of an earth failure or bad transformer nothing should effect both channels.
Ill take a look for free if we can find a circuit diagram for it.
Kuzma Stabi/S 12", (LP12-bastard) DC motor and optical tacho psu, Benz LP, Paradise (phonostage). MB-Pro, Brooklyn dac and psu, Bruno Putzeys balanced pre, mod86p dual mono amps, Yamaha NS1000m