My guess is that the channel switch, if it has one, might have some dirty contacts in it, the fact that the crunch channel works means that the majority of the circuitry in the amp most likely is working fine, so, you could try giving the channel switch a good spray with some electrical contact cleaner and work it a few times then see if it comes good, there might also be a bad volume or gain control pot in the clean channel that's causing all the trouble, I'd give all the pots a good spray with contact cleaner if I were you.....hope that helps.