Amp shut off and came back again.

It sounds like the power dissipation protection. Playing at a high volume level, the speaker load at that instant may have presented to the amp a (say) 45 degree phase angle which doubles the power drawn from the amp -- but only a quarter the delivered power is used by the speaker (i.e. there is no spike in volume). That would account for the very high heat sink temperature and the amp only shutting down for a second. If it was a thermal cut out, it would have taken longer for the amp to resume since the heat sink temperature does not drop that quick to reset the thermal cutoff device.

Thanks again.
l feel relieved.
My amp (Plinius SA 102) seems to get hot easily when operating in class A and pushed hard. It might need more breathing room too.

2" is insufficient room for cooling above most amplifiers! For a space that restricted I would use a fan for cooling. You need a good 10" to a foot above the amp- otherwise even though the heat might not trip the protection, the amp will be running warmer and things like filter capacitors really don't like that- they will have a shorter life.
Adequate cooling is important for any power amplifier.

I didn't realize it was a Class A amp, it really needs more space than that.

Mostly I listen in A/B mode, and the amp just gets warm, not hot.
Class A makes it generate lots of heat.
I'll get another rack or let the amp stands as its own, or maybe use a small fan.