I had an old PC with an Asus P8H67-V MB, which started shutting off under load, like gaming. I cleaned the MB and changed the thermal pads under the power transistor and chipset heatsinks, and installed a better cooler, Noctua NH-U12S for the processor. Put it into a new build with a Corsair CX550M power supply and a new SSD as boot drive. Now, I've tested it with two different GPUs (Nvidia GT640 and GTX960) and it still shuts down now and then, especially when running 3DMark or PCmark. It has 16GB RAM and memtest reports no errors. I'm seeing quite low temperatures of CPU, GPU and MB during normal operation. Does anybody have any ideas, or is the old MB just a write-off?