Intermittent problems are no fun to troubleshoot because you have to force it to fail or wait
for it to show the problem. It sounds
like it could be a thermal problem or a cold solder connection. Do you
hear the sound through the headphone
jack on the receiver when the speakers blank out? If so your pre amp section is
working, and it is in the power amp section. If not I would first use a spray contact
cleaner on the switches and rheostats. Click on the link below for Cleaner.
Second, wait until the
unit exhibits the problem then use a freeze spray selectively spraying parts of
the board and components until the sound returns. You can use a heat gun
sparingly to heat parts of the circuit to try to force it to fail. If you selectively
freeze spray the board and sound returns you need to inspect the board for
broken traces and touch up solder connections, which could be cold and faulty.
If you spray a component and the sound is restored it could be a bad component,
and it should be inspected. Click on the link below for freeze spray.