Computer shuts down after power on
I just put together a new PC. I've done a few of these before, and I have never encountered a problem like this. I've searched the web and can't find anything.
Intel C2D E7200
Palit Radeon HD 3870
OCZ DDR2 800 4GB (2 x 2GB)
Antec Neo Power 500w
When I turn on the computer, it starts up for about two seconds, then shuts down. About three seconds later, it'll start up again, then shut down after two seconds. Repeat. Everything starts running during these two brief seconds. The fans are all going, motherboard light is green, everything seems fine, then it shuts down.
During the past two hours I've removed the hard drive, the dvd drive, the video card, the fans, the memory sticks (tried one in each of the four slots), put in memory from the computer I'm on now (so the memory isn't bad), placed all the jumpers in every possible position, and of course reset the CMOS a countless number of times.
It might be worth nothing that when I started without any RAM, the computer stayed on for a few seconds longer before beeping and shutting down, and then continuing the loop.
I've tried everything I can possibly think of. I'm stumped. Can you guys help?