Hi Dan,

Have you checked that your 6M ohm grid leak is actually 6M ?

It does sound like a charge build up on the grid. Does the RF anode
current rise or fall when the radio goes dead ?

Are there any components attached to the grid that could be breaking
down slowly - capacitor, maybe even valve base.

Have you tried replacing the valve base ?

Have you tried a different valve ? Maybe broken or cracked valve bottom
(paxolin to valve pin).

Stick the whole unit in the oven (on low) and dry it out for 3 hrs -
moisture getting in somewhere ?

Does sound weird and interesting.

Regards,

Mark.