I've got an old Roland silver Cube amp. It's worked great for years, but now the volume drops precipitously at random moments shortly after it's turned on. After it's warmed up for 20 minutes or so, the volume drops cease. What could be causing this?
This kind of problem usually requires a repairman to open up the amp on the bench and start tapping parts to see what causes the amp to malfunction. If tapping parts gets us nowhere, then we start start measuring voltages and reading schematics to locate areas in the circuit that don't match voltages listed on schematics. needless to say, the amp must be malfunctioning when this step is being performed. Other items to check are defective solder joints, dirty or corroded connectors, problematic guitar cables, or even problems with the instrument. Be prepared to have everything tested.
As a electronic engineer who makes pedals/amps as hobby, I would say that mostly will be happening by aged solders and connections, or anything leading for a inconsistent polarization of the transistors. Probably, anyone with some electronic technical knowledge could help you remaking some soldering or replacing some transistors