Don't know about the Magni, but a friend of mine had the Asgard and he said it gets very hot, to the point that it burns your hand if you touch the chassis. Based on that, I'm guessing that's a common thing with schiit amps. Nevertheless, it shouldn't turn off. If I was you, I'd email schiit.
I have the asgard and if there is no air movment around it and its working for 30 mins+, its simply unhealty to touch it for longer than half a second.
I asked around and most people told me its ok, and that i should not worry.
However i did get it a laptop cooler just to be safe. In my opinion, anything running that hot is bad in the long run.