Over the past month or two my PC has been shutting off for about 5 secs then restarting. It occurs approximately every week give or take a couple of days.
I've cleaned it of dust, checked the temps (all under 50C when the shutdown occurs), updated BIOS, checked for viruses, passed memtest86.
Gigabyte GA-X58A-UD5 rev 1 Motherboard
Core i7 975 EE Quad Core @ 3.33 GHz (not OC'ed)
6GB (3x2GB) Corsair PC-10600 1333MHz DDR3 RAM
GeForce GTX560 - 1GB GDDR5
Antec 1000W CP-1000W
Microsoft Windows 7 Professional