Thanks for the additional information Spaceatron
I suspect that the PSU may be the cause of the issue as it does not maintain the consistent power supply that the card requires, this based on the fact that Gigabyte show a 360W continuous power supply is available, a maximum combined wattage of 450W, a peak wattage of 550W and then to justify these figures they make the following statement regarding the PSUs performance;
* Advertised performance is based on maximum theoretical interface values from respective Chipset vendors or organization who defined the interface specification. Actual performance may vary by system configuration.
The maximum load on the +12V rail is shown to be 360W on the PSU database, see here
Please read the first page and the conclusions page here
where the GE-450P-C2 version of the same PSU was independently tested, not good at all
What I would do first of all is uninstall the video cards drivers, shut down the computer, remove all power cords from the PC, remove the video card, reassemble the computer, connect your screen to one of the MBs own video ports, restart the computer and see how the video/display performance is.
Let us know how it goes.
You are welcome btw and I am sorry that the reply is rather grim.