Why shutting down?

Check speaker voice coils.

I might try unhooking everything and see if it still does it and also running it in a typical temperature environment (how cold is it in your shop?). Likely something internally that the protection circuit doesn't like so you would need a tech to look at it.

May be your amp stands in damp/wet air?

I have a newer Yamaha in the shop now because it was shutting down. Waiting for an answer from the tech. I am guessing thermal protect circuit.

It really depends on what the protection circuit is sensing. If it's D.C. at the output, it could be a leaky output transistor. It could be as simple as the protection circuit is set overly sensitive. If you are using very old speakers with foam surrounds, it could be a woofer voice coil rubbing causing an intermittent short thereby triggering the protection circuit. Try another pair of speakers.