[Username1]

On those control pots, I don't have one of those sets, but he has had this problem for
years, and probably played with those controls much more than they were ever meant
to be used in their life... They could be really in need of cleaning, or if they are PC board
based the solder connections to the board could be faulty...

Also that cap you guys just replaced was not subject to that much of a high voltage
while it was working.... 200V ~ There may be other caps same manufacturer that can
be getting ready to go.... Blanking, or Video Amps.... Coupling caps....

Does the set tend to blank-dark, or go white and cut the HV that way....?
I guess it would be a good thing to also check to be sure all that other stuff
around the flyback is all connected, nothing left unsoldered...

If you leave it alone for 20 minutes with a show on, does it do it on it's own
and can you point to specific types of things that cause it...?

Did you replace that 6GM6 that also had the DC added to the filament ??
Might be a good thing to do, just temp swap it with his other set....

.