I have a Technics SA-GX505 amp/receiver that runs fine for a while, shuts
off for a few minutes, and then will come back on, etc. It tends to run
longer at lower volumes before shutting off; however, no components appear
to be getting hot, not even the main power amp. And the cooling fan works
fine, which is driven from the speaker output level.

I have the service manual which is a little help, but nothing looks obvious.
It's not the output relays that are kicking in, but the main power supply
relay. It's RL 751 and its driver Q751 are driven by RLY (pin 27) on IC 901
the main microcomputer. So it appears the protection circuitry is working, I
just don't know what's triggering it. Any ideas what to check? Speaker
impedance is OK, and again, no obvious heat issues. In my experience, most
of these problems tend to be with the main power amp (IC 601- SVI3206C).
Should I just start there; any other ideas?

Many Thanks, -Bob