I bought this used a week ago (very cheaply - PO said there were problems with it). For the first few hours of use, it worked fine. Then it would run for a few minutes, and begin to crackle and cut out. Now it'll only run from cold for 30 seconds or so. After it cuts out completely, you can still hear it at a very low volume, and it sounds more or less the same as it would normally. Just extremely quiet. Here's what I know:

The tubes don't seem to be the problem. I've swapped them all out with no significant change.
The speaker works fine.
Cord works fine.
The amp will run for longer if I play very gently, and will cut out instantly if I hit a chord or pick aggressively.
Tubes don't seem to be running excessively hot or glowing unevenly. Rectifier tube is RCA NOS and glows a bit more than I'm used to seeing in other amps - about as brightly as the power tubes.

I'd much appreciate any help you can offer.

Do NOT use a pencil as a probe and make sure that you are sitting on your other hand.   There are lots of very lethal voltages in there. Pencils are conductive.  If your other hand is grounded and you get bit, the current will travel across your chest to get to your grounded other hand.  Not good for the main pump.  I like PF Chang's chopsticks best. ;-)