I've seen this fault, and it was an open circuit on the gun ie. no
return ie.
it was floating. Initial potential was ok, however a flashover raised
the potential and high beam current, took a long time to leak away.
I'd look for a crack on the neckboard, and a check for any feed
resistor gone open. (don't measure with component in-circuit)
Clean down the board with alcohol.
And look for the discharge in a darkened room.

Pedro