So what power amp is on the back of that thing? Is there a row of transistors under black plastic covers across the rear? Or does it have them inside? 400 series? 400B/G? 400BH?
Immediately shut down? usually it takes a little while for heat to build up.
What exactly happens? The entire amp goes dark? Have you verified that thermal breaker is indeed what is opening? The heat sink has to get darned hot to open one of those, so is the thing too hot to touch? The thermal breaker kills power to the entire amp.
If that is what is happening, then you need to determine if the amp is really overheating and the thermostat is just doing its job, or if somethig else is the matter and the amp is shutting down but not really overheating.
Here is one for you, you crank it up to 50% and it shuts down. How long does it take after you remove the signal for it to come back on? If it is thermalling, then it takes time for the heat sink to cool.. If it comes back immediately, it doesn't sound like thermal.