Hello Ed:

I had a similar problem and I kept looking in the wrong spot. I assumed it
was a leaky cap and replaced all the caps and tubes. Finally, out of
desperation, I convinced myself it had to be something else. About that
time I was reading about resistors. The carbon composition resistors are
often absorb moisture and change resistance when heated. Changing to a
newer more stable resistor solved the problem. I think there is a chance
you are affected with the same problem.

73, Colin K7FM