Look for bad solder connections on both circuit boards. Common places are the input jacks, the controls on the front panel, and any heavy or hot-running parts. You would not believe the number of amps that have only that problem causing all the trouble.
Also, before you start any repair, operate the anp with a constant signal from a radio, etc. Tap around with a wood dowel, plastic stick, or some other non-conductive tool. Chances are good that you can find the problem area like that.
Mike.
Mike, Amp Repair Guy, April 2007