[Mondial]

I doubt its the output tubes. If the volume returns when you touch the cap of the RF amp 6K7, then it probably not related to the output stage. Does the B+ power supply voltage change when the problem occurs? Gassy outputs would make the B+ voltage drop noticeably.

Sounds like it might be related to the AVC circuit which controls the negative DC bias voltage on the 6K7 grid. This AVC circuit is also connected to the 6A8 and the 6K7 IF amp, so I would also try replacing these with known good ones.

You can also check the resistors marked 7 and 34 on the schematic and see if they have increased significantly in resistance.