A couple of things come to mind here....
Although the PSU appears to be single rail, a good thing, if the age has caused a power dip, we would swap in a known good PSU for testing.
Also, because the driver match for the operating system, motherboard, and graphics card is critical, be sure to update the motherboard/chipset driver and BIOS firmware, plus check for updated drivers for the graphics card.
Hope this helps.
I have an i7 920, 8gb ddr3 RAM.. Novatec 750w 80plus PSU. Everytime I load a game or stress test the gpu it locks up the whole system and I have to hold the power button to restart.