Problem with PCI-E Graphics Card

Hi all, I have recently fitted an Nvidia GeForce 7900 GTX graphics card, it's second hand and has two dvi ports on the back and my computer is running it fine, but because my monitor only have a VGA connection, i have had to use a vga to dvi adapter; just like one of these from Amazon: http://www.amazon.co.uk/Cabling4Less...3510681&sr=8-1

But when i my monitor is plugged in through the graphics card it begins to show little red, green and blue pixels in random places on the screen, however this is only when it's showing a black colour that these pixels show up, it looks as if its the graphics card that is showing these; because when i plug monitor back into my motherboards vga port these pixels do not appear.

I have also tried using two different monitors, two different dvi to vga adapters and two different vga leads, all of which still show the same fault.

Also i have installed the latest software and drivers from Nvidia's website.

By the way I am running Windows 7 Home Premium 64 bit

I am wondering whether it is the graphics card which is causing this, because this has been my first ever pci-e graphics card that i have installed so i'm not sure why these faults are occurring, I'm just wondering if any of you guys know why?

Many thanks, Ryan.