Unexpected shutdowns are usually due to 2 possible causes: A bad power supply or CPU overheating. Since you tried 3 different supplies its unlikely that the PSU is causing this. So, you can monitor the CPU and case temperature using Everest Home
. It also will tell you all sorts of information about your system.
If the CPU is overheating the mobo may be going into emergency shutdown to keep the CPU from frying. Check your BIOS for the shutdown temperature. Set it to 60C if you can. Prolonged high temperatures will shorten the life of CPU.
Are all the fans running ? Check the fans on the CPU, case, Northbridge and video card if they have them.
If the case temp is OK (35C or less) but the CPU is overheating then there are 2 possible reasons: 1) A bad thermal contact between the heatsink and the CPU; or 2) the heatsink/fan is inadequte for the CPU. I recommend removing the HSF, cleaning it with rubbing alcohol and a soft rag or tissue, and applying Arctic Silver 5
(a small amount spread with a credit card). If this doesn't bring the CPU temp down then you're in the market for a new HSF cooler.