Video Card issue

Hi,

Have a perplexing situation here.

I'm currently running SLI on an 850 WATT PSU.

I run 2x Leadtek PX8800 GT video cards in SLi mode, for the first month or so everything seemed fine, I then reformatted my pc with same same O/S with all the latest driver updates (VISTA) and found that only one video card seemed to work.

Now, I've run off the theory that the other PCI-E slot may not work, so I swapped the cards over - the faulty card still didn't work while the other card was fine - keep in mind I also swapped the PSU connectors as well - no difference.

I then brought the card back to the shop and asked them to test the card and if it didn't work to have it sent off, a week and a half later the guys said it worked fine on their system.

I then tried the card on my friends system and it worked fine seemingly, i then popped it onto my motherboard and still NOTHING!

I am at my wits end - it has effected my psyche that much, that I'm ready to buy a MAC.

Now that's saying something.

Kind Regards,

Nick