Computer Restarting Upon Startup

[HMZinc]

Hello everyone. I built my computer back in February of 2014, and it has been working like a charm, except now. When I turn on my computer, it will stay on for a second, turns off, and then turns back on and works fine. I have tried multiple diagnostic tests, and I know that it is not the memory, video card, or any of my hard drives. I have yet to update my BIOS, but I will be trying that. Could it possibly be the power supply? It has never shut down on me before, and this is the only issue that I have had with my computer. Any help would be great. Thanks.

BUILD:
CPU- i7 4770 @3.4Ghz
COOLER- CoolerMaster Hyper 212 Evo
Mobo- Asus Z87-Plus
RAM- 16GB GSkill Ripjaws X @1600Mhz
GPU- EVGA GTX 660SC
PSU- Antec NeoECO 620W
HDD- Western Digital Caviar Blue 1TB
OS- Windows 7 Pro 64bit