Matter of fact, an intermittent/faulty thermal sensor(there are two, controlling the fan) or thermal breaker(also two), was also my first thought. Does the pilot lamp(in the power switch) blink when the amp is acts up? This is the thermal overload indicator. Does the amp have adequate ventilation? Have you ever cleaned the heatsinks? The fan can deposit a lot of junk on them in 30+ years. Does the fan go into it’s high speed mode, prior to shut down? However: If it’s an overheat issue, it would take longer than a few seconds, to return to normal operation. Any possibility of oscillations in the input signal? Those will shut the amp down, also(triggering speaker relay). Does that happen, when listening to all your sources, or just one? Another possibility: the power switch’s contacts are getting weak, from the high currents at turn on. Those were a common source of trouble. Hafler used a beefier switch, in the later runs of the amp. Does the condition occur during listening, or at idle? DC offset, at the input, will cause the speaker relay to open. If you have an earlier iteration, capacitor C1(on PC-19) will be an electrolytic. Those were replaced with a polypropylene, in later models. It’s possible that those(if electrolytic) are now dried out/faulty, and allowing DC to pass, opening the speaker protection relay. Then again, the relay itself can be going bad, or something in it’s associated circuit. Can you hear the relay clicking, during the events?
