GA-MA790GP-UD4H from Hell

Hi. I just put together a new system. The 1st mobo was DOA. IGP was defective.
I returned the mobo, they sent me the new one. Different serial #.
And now got the Cold boot.

The problem is that the system never boots or POSTs on the first try. But It does boot correctly on the second try. When the system is cold and has been off for some time (more than an hour) it will not boot. I have two power switches the "soft switch" on the front of the computer case and the "real" power switch on the back of the power supply. Here's what happens: I turn on the real power switch, then press the soft power button. System turns on, fans spin up, LEDs light. But there is no beep from the motherboard and no video signal, even after waiting 10 minutes. The hard drive and CDROM blink their lights. I press the reset switch and the drives re-initialize, but the system never boots. The front, soft power button does not respond. I can only power off with the rear power switch. So, I power off with the real power switch. I wait 30 seconds and then turn the computer on again. The computer then POSTs and boots normally. After booting, the system appears perfectly normal and stable. This is a consistant problem. It happens every time I try to turn on the computer after it has been off for a long period of time.

More details:
Vista Ultimate Sp1
Gigabyte GA-MA790GP-UD4H motherboard,
AMD Phenom II X2 550 BE CPU,
OCZ 2x 2GB DDR2-1066 Platinum Ed,
OCZ StealhXtreme 500w
2 x 120 mm cooling fans + x3 80mm
CPU has the stock heatsink and fan mounted.
The system stays between 36c to 40c
I've run Memtest86+ for hours with no errors.
I've tried booting with only one memory dimm.
All BIOS settings are auto/default and seem reasonable.
I can't think of anything else.
This problem is too weird. Is it the motherboard, CPU, memory? Or something else?
Another RMA?
Going Nucking Futs!
Thank's in advance,