System sometimes booting with XFX 7970

Hi all,

I've installed a XFX HD7970 in my rig a week ago, but it is having some problems booting up. My components are the following:

Mobo: Gigabyte P55 UD3R
CPU: Intel i7 860 @ stockspeeds with a Zalman cooler
MEM: 4x 2gb Corsair XMS3 @ 1600Mhz
HDD: 1x 256gb Crucial m4, 1x 1tb Samsung spinpoint fsometing
PSU: ANTEC Truepower 650W (almost 2years old now)
Old GFX: XFX HD4770

My rig will only boot up sometimes (less then 10%) when I install the HD7970 in it, but it will boot 100% with the old HD4770 in it. Once the system has booted once with the HD7970, then it won't have any problems with rebooting or turning the system on and off again. So this problem looks to be only occuring on a cold boot.

I'm not really sure what the problem is, thats why I come here to ask you guys for your opinion.
My current guess is the PSU, but from what I've heared 650W should be enough for the HD7970 in my current setup due to it's low power consumption.

What's the exact problem?
With the XFX HD7970 in my rig, the system won't startup when coldbooting. The problem has occured once in a reboot, but the other scenario's are coldboots. Not starting up means that all the fans (cpu & gfx) will start, my Samsung HDD starts to make sounds that he's reading, but I do not get a signal on my screen nor I will get any sound that Windows started up.

My screen shows black all the time, like it's done when the system is powered off (the GFX doesn't output anything). It looks like the MOBO isn't starting up.

Things I've done
First of all the installation:
I've installed the HD7970 in my 16x PCI-E with a 6+2pins PCI-E connector and a 6pins PCI-E connector comming from two different rails out of my PSU. Installing the card in my other PCI-E slot (only 4x) doesn't make any difference. Neither changing around the PCI-E connectors, e.g. connecting 6+2pins and 6 pins from a single rails or switching connectors around between the rails.

BIOS:
Updated the BIOS to the latest possible version found on the Gigabyte site. I actually didn't do this before so my SSD was not trimming *oops*.

Drivers:
Updated all the chipset drivers. Was able to do this since the system boots normally with the HD4770 in it.


What do you guys think that might be the problem and how I can resolve this?