Sounds like a loose intermittent connection in the amp that is only being triggered by the loudest of vibrations. Try injecting a signal into the amp while giving the cabinet a nice whack or two with your fist. Does this trigger anything? Take the chassis out of the amp, inject a signal and use a chopstick to move around specific connection points. Does this trigger anything? Look at the schematic and locate the areas on the board that comprise the power supply sections. I would chopstick those areas the most to really get at it. Does the light on the front of the amp go out when "it's loosing power" or does the sound just cut out?