Peavey Bandit 112 Red Stripe issues

Hi guys.
I just got a Peavey Bandit Red Sripe (1995 or later) for repair. I received only the amp without speaker encosure.
Acording to the customer, it suddenly stopped working, only a very faint sound is comming out of the speaker.
I checked the power amp and it had an open diode (SZ-13886). I replaced it with a 1N4007 and the power amp works.
The preamp section only works after messing whith all controls and switches, and then wacking it a few times...
Once it starts working, it sounds just like expected.
It has the notorious "IC" brand e-caps in it. None of them looks bulged or otherwise suspicious, though.
I suspect a cold solder joint or in the worst case a broken trace. This preamp pcb has traces on both sides. But it also could be related to an electrolytic cap.
Any ideas what might be going on?