It's unlikely that every tube socket has the same malfunction. But to be sure, I'd take 5 minutes, spray each socket with Deoxit, and work the tubes in and out a few times. Then I'd take a dental pick, and retension the socket springs (just use the pick to mush the metal closer together). Then put the tubes back in and fire it up.

If you still get dropouts, etc, I would look hard at the filter caps and resistors. The big stuff, related to power supply. Chopstick them hard, or wiggle them a bit. (be careful of course). You would be looking for a cold or busted solder joint.