Check speaker voice coils.
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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.