I think you answered your question when you said "old". If the amp is truely old... then there are a number of things that it can be. With newer amps, if you experienced the same thing, you would expect it to be a problem in the tubes. With older amps... other things can come into play. Such as bad pots, faulty connections, and bad grounding connections. Check the wiring to make sure it looks fairly clean. If it looks pretty ratty... then your problem might even lie within the wiring.