[Chuck Schwark]

Obviously the 6Q7 needs changing, but also you say you
checked resistors, but did you replace even the ones that
checked "good"? A resistor can drift off/up once current
is flowing thru it. I, as a general rule, do not trust older
resistors any farther than I can throw them. I would
replace them to eliminate one variable.

Also, check the 6Q7 socket pins and a possible "open"
intermittant coil in the last IF can. Tapping around
with a wooden pencil and tapping wiring in that last
IF and DET stages may help.

Typing out loud,

Chuck