ok so heres my problem. ive had my computre for just over 2 years now. never had a problem with it til recently. when i turn the computer on it starts up, all the fans and everything kick on, but after 5 seconds the computer just powers down, waits a second and then tries to restart itself. ive narrowed the problem down to when i unplug the 8pin 12v cord from the mother board the computer stays running but when i plug it back in it gives me this problem. any suggestions...

What computer is it?

its a custom one i built. heres the specs;
-GIGABYTE GA-EP45-DS3R LGA 775 Intel P45 ATX Intel Motherboard
-Antec EarthWatts EA650 650W Continuous Power ATX12V
-Intel Core 2 Duo E8500 Wolfdale 3.16GHz 6MB L2 Cache LGA 775 65W Dual-Core Processor BX80570E8500
-HIS H487F512P Radeon HD 4870 512MB 256-bit GDDR5 PCI Express 2.0 x16

those are whats in the computer if it helps at all

It does. You need to have a close inspection of the capacitors on your motherboard to see if any have domed up and/or leaked. If you find any, they need to be replaced.

all the capacitors look good and none have leaked. ive tried taking out the mb battery and waiting 30 mins to put it back in. i went to try another power supply but it wasnt compatible.

Have you tried running it with just 1 ram stick? You could have a faulty one perhaps?!?