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I assume you've checked the obvious, like cleaning the tube pins, wiggleing all the tubes, etc... If this was a solid state set, I'd suspect bad solder connections on the board, but since this is a tube set, it shouldn't suffer from thousands of bad solder joints. But, if this set has been serviced a LOT like you mentioned, I'd check tube sockets for damage, wear, or cracked solder joints underneath the chassis, and make sure that none of the sockets have cracks in the bakelite/plastic/phenolic/whatever or show signs of arcing. Also, it could still be a bad tube - even if a tube tests good, it might not work great in a specific circuit - same with weak tubes. Some tubes may test weak, but work perfectly in a givin circuit, i.e. a weak 12AX7 will work perfectly as a phase inverter tube, even though it might not work as a preamp tube.
Also, try gently tapping on things with a plastic pen or something while the set is running, put a big mirror in front of it so you can just look around and see the picture in the mirror. Tap on controls, tube sockets, connections, etc - it could be another dirty pot in there somewhere.
I've had stuff with intermittent, nagging problems before - sometimes it's just best to sleep on it, and come back the next day, or have someone else come over and look at it, sometimes a fresh pair of eyes can help.
If problems persist, you might need to take on more drastic measures, possibly involving goat's blood and candles.
Good luck!
-Ian
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