I have a 850w psu that has 16amp on dual 12v rails.My power usage monitor shows that the pc is using 520w on idle and 640w on full usage.My problem is that when i run any 3d app for any length of time, the psu shuts down and reboots the computer.Ive checked the psu with my tester which says it's fine, checked wattage and temps, again fine, and cannot fathom why the psu would act in this way.Does anybody have any idea's on what could be causing this?