I think everyone is overthinking this one. First check to be sure that the protection relay is good. If the unit works longer with higher volumes, the relay is usually the cause of the cutouts.

overheating.

Check for frayed and potential crossed wires at the speaker connections. A single strand may not be enough to cause the short detector to kick in, but enough to bring down the impedence to make the amp work too much.

Given the age of this unit, I would start by checking the entire unit for poor/cold solder joints. Continued use of this with heatup/cooldown cycles will crystalize these connection over time. Look carefully at the end joints at the display. This is a common area for poor connections.

Try these and post any updates.

Dan