Ditto the overheating. The guy at the shop probably doesnt stress it like a 3d app does. Running without a cover doesnt help because the air is no longer moved through the case - imagine trying to heat your house by blasting the hot air in the basement without any ducts to transport the heat through the house. Now you could try running with a room fan aimed at the motherboard and see if that tells you it is indeed the heat causing it to shut off.  If it is - as pointed out - upgrade the cpu cooler and case fans.

I'm a bit rusty at the PC fixit biz - its been a long time - But from what I have noticed - cooling is even more important with these later generation rigs than it was when I made my nut fixing the things.