PC won't boot... power issue or mobo fault

my PC did not power up when I pressed power button. The motherboard has lights (leds). turning power on at psu lights up leds. pressing power on pc button fires up the fans for < 1 second and then nothing.fans stop, boot process doesnt proceed but lights remain on board.

So I buy a new psu (another 800 watt). I unplug all but bare essentials on board. same issue. I then take out the 8 pin adapter (so only the 24 pin is connected) and leds remains on and boot up starts (yipee). I then put the 8 pin in again and it fails in the same manner as before.

Gigabyte GA EP45 DQ6 mobo
4 hard drives
2 dvd
intel q550 ( I think)
4 x 2 gb RAM
800 watt psu

any idea where fault lies? I assume board.
 
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