Doby

Seeing as it just flat out shutsdown without any bsod suggests that its either the psu, heat or motherboard.

Running a cpu burn in program for a hour without shutdown tells me its not heat.

Yes the psu is sufficient but I wonder if its not flaky meaning it drops voltages on one of its rails at any given time causing the shutdown.

I would run the cpu burn in for a full 24 hours to see what happens and to really stress the system or prime95's torture test for 24 hours with max meat and power to see what happens, all the while monitoring the voltages and temps with software to see how stable the voltages are