unable to boot up

hi guys, i posted about a month ago with a problem, my pc wouldnt boot up. at the time i thought it was the processor because when it was removed it booted up, i had it checked and it now turns out that the processor is fine but it still wont boot up. i did a little more testing and noted down the post code display that is located on my motherboard and the problem seems to be with the 4pin 12v ATX cable. i am using an Abit AT8 32X. the post display is as follows.

ATX Cable attached, cpu also installed:-
(8.3.) - ATX power supply ready
(9.6.) - De-Assert cpu core voltage
(9.9.) - Complete power off sequence
i try to boot the pc but it switches itself off withing a second.

ATX Cable attached, no cpu installed:-
(8.3.) - ATX power supply ready
(9.0.) - complete uGuru initial process, AWARD BIOS takeover booting job
PC boots fine into POST screen although i have no display.

when the ATX cable is removed it obviously boots until post then shuts down, this is indicated on the POST code display.

i have just bought a PSU tester to test the ATX cable, so assuming that i will find no problems there do you guys think that the problem lies with the motherboard? if so any indications on what it might be? i dont want to fork out £100 for another one and it turns out its not that. also ill add that my pc was running fine for several months before this happened and i have made no changes since. thanks for your time, if you need any more info i can provide it.