GA-EX58-UD4P strange power up problem

I have a strange problem when I power up my system. Once I push the power button, the system powers up for about 2 seconds and then shuts down for 3,4 seconds and then power up again, and everything is fine from there. This only happens when I have a real power up. It doesn't happen if I restart or wake up the system from sleep mode. I wonder if this is a MB problem.
Here is my system:

Gigabyte GA-EX58-UD4P Bios: F7
Intel i7 920
6 GB OCZ XMP running at 8-8-8-24 1600MHz
ATI 4770
Vista 64bit.
No OC on anything yet. I also disable the turbo mode on the MB.

Any suggestions?

What PSU do you have?

I have the same 'problem' with a couple of systems. Been having those symptoms for more than a year. I read on the ASUS website it is normal. But then I swapped out the PSU in one system and it stopped doing it, powering on then shutting down briefly and then power up correctly. Check all the connectors from the PSU to the MB/hardware. I am thinking it is a ground problem of some sort. My 700w Thermaltake still does this. I bought a 750TX to replace it when I have time.

One thing I haven't pointed out in quite a while, that can be responsible for all manner of really odd problems is:
don't use insulating washers to mount your MOBO on the standoffs; if you examine the board, the area around the holes has a radial array of solder pads - these are part of the systems' 'grounding solution'; all of the signals (busses, etc.) that are 'captured' by the board's subsystems are done by comparison to a (hopefully, firmly 'fixed') ground level; if that ground level is 'wandering around' on various parts of the board, it can cause bizarre havoc... Also possible then are 'gound loops' - circulating currents through the ground circuits that also raise cain!