Hi,
The symptom suggests an intermittently failing power supply component, an intermittent solder connection, a hairline crack in the PCB trace,
or infrasonic oscillation due to the output-stage-to-V/A-stage feedback through B+.
You might want to examine the PCB traces for hairline cracks, especially those of the ground and the B+.
You might also want to re-solder all the joints, wires, lugs, etc.
Then, the list of usual suspects:
1. Replace the fuse.
2. If there is an inrush current limiter, replace it.
3. If there are any "snubbers" left, take them out.
4. Replace the diodes. Both of them.
5. Replace all the power filter caps, one by one.
6. Replace all the power filter resistors, one by one.
7. If there is a choke, replace it.
8. And, finally, replace the power transformer.
After all this, it may just turn out to be a bad power cord or a flaky switch...
Or you may have to hire an exorcist for your speakers...
Good luck!