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?

Sounds like a thermal break somewhere. I would take it in or if you are so inclined start checking circuits for resistance. Also look for swollen capacitors. One last thought is maybe the power supply is done.

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.

In older Yamaha integrated units that used the output MOSFET based ICs they would often get separated from the heat sink. You can see heat sink compound shipping off and even mounting screw is loose(often due to Chinese manufacturing defects of automated production).
I would unscrew heat sinks, clean them and output devices and than connect them back together reapplying again heat-sink compound.
I would also suspect insufficient heat sink area pathologically in many Yamaha units. Try to place fan next to chassis and see how it will behave. Some of the units have fan placed next to head sink area that also may malfunction and cause such shut downs.
There are connectors inside(just like in PC) all Yamaha PCBs that will allow you to connect a fan as another option.

Yamahas also shut down when they have unmatched impedance as well.
MOSFET ICs used in Yamaha require speakers not to dip bellow 4Ohms.

you have to check 3 things: Ac power, speaker wiring or Ac problem.
Disconnect speakers and see if the same happens.