Relevant Specs: Sapphire 5850, core i7 920, Seasonic x750 power supply, ECS X58B-A2 mobo.

From a cold boot, meaning all power totally disconnected at the wall, the system will power on for less than a second and then power off completely. After two seconds of being powered off this will repeat and on the third "attempt" the computer will boot.

To be very specific here is what happens:

1. The power is turned on at the wall and on the psu, the orange stdby LED on the mobo is illuminated but the system is 'off'.
2. I hit the power button on the case or on the mobo itself
3. I hear the relay (?) in the psu closing
4. The case light comes on and the mobo power light comes on.
5. The fans start rotating.
6. Immediately after this the I hear some relay click - the power lights extinguish, the fans stop, the stdby light remains on.
7. Less than 2 seconds pass and the cycle repeats without any intervention from me.
8. On the third attempt it boots normally and the machine runs perfectly.

If I do a soft reboot or a full shutdown the computer starts normally the next time. It's only if I pull the power cord or flick the switch off on the PSU that I get the cycling again. Basically any time the stdby light on the mobo goes out.

I have removed the graphics card and I get the same problem.

I have removed the PSU, hotwired it to the ON position and verified voltages on all lines. The relay does not cycle when I do this.

If I connect only the 24 pin ATX connector to the mobo and not the 8 pin ATX12V / CPU connector then I will not get the cycling, the fans run, the power light stays on, but obviously the system can't boot.

Disconnecting all fans has no effect on the problem.

My feeling it that it's something to do with the motherboard like a capacitor that's taking a long time to charge because it's leaking or something along those lines. But I can't imagine what could be 'wrong' with it and only manifest itself as a problem under these very specific circumstances.

Any ideas?

Thanks.