I found out that an isolated tube heater winding, over an hour or so,
makes the tube focus drift a lot.
I connected one end back to ground, and focus has stabilized.
A strange thing happened.
The thing is, there are no more green flashes after a few days, since connecting back up.
I tapped the neck as before, but nothing. Maybe it wasn't a H to K short after all?
I'm not sure whether to be red faced from embarrassment or not.
Possibly my analytical approach was incorrect.
Originally tapping the tube neck, in the past would also have given the pcboard attached to the neck,
with all it's components and solder joints, vibrations in unison.
I guess a real technician would add a temporary wire harness from pcb to tube neck,
to isolate any vibration. I don't believe the video amp was at fault.
They are analog. They either work or don't work, and they sure don't heal by themselves.
So there may have been a bad solder joint, that is now fixed.
Anyways, still happy, never spent a penny, only time, and only time will tell.
Still thinking. Could have been one of those random neutrons from cosmos,
or alternate universe glitches! Nah.