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.
Why shutting down?
I have a Yamaha A/V HTR-5890, runs from a few seconds to 2-3 minutes, then clicks off. I have had it for a couple of years, no changes. I run a 10" Klipsch sub, Chorus Klipsch speakers, along with some older duel 12" MTX's, and a older smaller Advent's for surround speakers. It has never run hot, this is my shop system, so it's colder out there then inside.