CatlinPM

On starting up my computer I have suddenly been experiencing horizontal lines appearing on my monitor. The computer will go as far as the windows screen and than the monitor goes black. I've tried switching out the monitor and still have the same problem. I know that usually points to the video card being the problem, but here's the thing... I have 2 video cards, both BFG Tech GEForce 7900 GS OC cards set up in an SLI system, or it was setup that way I should say. When the lines first started I thought it may be the first video card that the monitor was plugged into, so I completely removed it and transplanted video card number 2 into the primary slot. That worked for awhile, than the lines began on the second video card. Now neither cards will work in the primary slot, when I put either card in the secondary slot however there are no lines with either card, but I get a message at startup to move the card to the primary slot. I have also tried putting both cards back in and have actually gotten the same message saying I need to move the secondary card to the primary slot. This leads me to believe that the video cards aren't the problem, but that there is something wrong with the primary PCIE slot on my motherboard or with the motherboard itself. I have a Evga 680i motherboard by the way. I really need some help figuring this out because it is driving me bonkers and I need my computer for business reasons. I have also made sure to blow out any dust in the computer and/or slots very well with compressed air so I know that isn't the problem either. I also checked for any loose connections and have reseated everything...Any help would be greatly appreciated!