Strange V3 problem

[shakti]

I decided to post in this section as well, since the other thread was getting rather lengthy and cluttered with info that wasn't really relevant to the problem.

I have a JTM45 build with a RS Deluxe OT clone which was working perfectly for 2 years. Then I started fooling around with trying different primary impedance and secondary impedance etc...long story short, I'm back to the starting point (8k primary, 16 ohm secondary). However, I now have a very strange problem, which may or may not be related to my fiddling around...

Occasionally and intermittently, V3 flashes up like a firecracker for 3-4 seconds at a time. At the same time, the sound cuts and there's only a loud buzz/hum. The problem resolves itself after a few seconds, and the amp then works perfectly. So in between these episodes, the amp works perfectly. I've had the board in and out several times, reflowed V3 connections both on the board, the wires underneath, and on the socket. I've replaced the V3 socket. Tried several sets of tubes. Nothing seems to affect the issue at hand. The problem is intermittent, so it's hard to get any good readings of what is going on when it happens, but no fuses blow, and the tubes seem to survive just fine. I can't see any sparks or arcing on the socket itself. I can play anywhere from 2 to 30 minutes before the problem occurs, then anywhere from 2 to 30 minutes can pass before it happens again.

Any hints or clues? I'm really in the dark here...
If I get lucky, I may catch when it happens on a video and try to post it on Youtube.