Sounds like tubes or power source to me. Tubes will do that. Good luck! Don

Let me start by saying, I would recommend that you take this to a repairman!!!!!!!!
However, for my 2cents worth.
Sounds like when the amp gets warm, you develop what I would call an "open short". From the nature you're describing, would probably suggest the preamp section of your amp. This can be caused by a multitude of different things, from something as simple as a wire going bad, or bad input jack to something a little more complicated, like an IC in the circuit.
This can sometimes be narrowed down by visual inspection of a clean board, by looking for black burn marks around any components in the circuit or melted insulation on caps, resistors, or wiring. Other times you need to get your test equipment out and check each component to ensure it's operating withing the given parameters (ie, metallic core resisters have a tolerance of +/-5%). You may also need to put a small amount of current through the circuit to find the problem, something like a tone generator that they use for phone lines, and find where the current stops. But I would probably leave this to a professional.

As always, with troubleshooting, start with the easiest and cheapest, then work towards the harder problems and more expensive components.