I would feed the amp a signal from a generator or ipod or phone, just not a guitar because you will need both hands. Turn the amp on and while it is cold probe around with a chopstick to see if moving, tapping, flexing or putting pressure on something makes it get louder suddenly. Include the speaker wires if they are not soldered. Check the speaker jack and plug connection. I had a Victoria 410 Bassman do this once, it turned out the speaker plug was faulty. Rock all tubes to see if there is a dodgy or oxidized socket. Check input jacks, does it do it on both, does it do it on both the Normal and the Reverb channels?