My friend has a GK 400RB bass amp that the audio will cut out on sporadically. We'll be playing and then the sound will shut off. Powering off and then on again will usually bring it back. It isn't losing power. I've done some basic troubleshooting on it. Checking voltage per the schematic, etc... but I'm at a loss as to where to look next. The boards and traces look good. No obvious bad or burned components.

I haven't been able to recreate the problem mainly because I don't have an opportunity to play for a while to make it happen. I'm also much more familiar with tube amps so my ss knowledge is a little weak.

Any ideas as to why the audio would cut out and require a power cycle to work again?