I had to resort to my old 2 GTX 560 Ti's as my GTX 780 has failed and must be returned. At first, Sli had not worked, but after several updates and installations on my Asus P8Z77-V motherboard fixed the problem, but only temporarily. I come back the next day to realize that Sli had stopped working, and hasn't worked since. Both cards are working perfectly, as they are being detected in the Device Manager, and I swapped each other to test each one, as well as swapped Sli bridges. Under the Device Manager, I was given the Code 43 for one of the cards. However, once I disable it and re-enable it, it starts "working" (quotes for uncertainty), however Nvidia does not recognize Sli when Windows 8.1 is active as far as I'm concerned. When I reset my computer, I get the same thing all over again. I have tried countless re-updating, swapping cards, PSU cables, and my motherboard is as healthy as the day I bought it. My Windows 8.1 is fully updated and my motherboard is flashed to its most recent release. Can someone help me out?
Corsair Vengeance 8GB DDR3 1866 MHz Ram
Ultra X4 1050-Watt ATX Power Supply
Samsung 840 Evo 256GB SSD
Seagate 1TB HDD
Western Digital Black 1TB HDD
Ivy Bridge Intel i5-3570K 3.4 GHz (3.8 GHz Turbo)
2x Galaxy GTX 560 Ti 1GB DDR5 Graphics Card (Sli'ed of course)
Asus P8Z77-V LGA 1155 Motherboard
Both graphics cards are the exact same model (You may check it out for yourself here: http://www.newegg.com/Product/Product.aspx?Item=N82E168...), have both been used for Sli in the past extensively (and still work fine to this day), and have never been overclocked. Both cards are properly secured as well as the Sli bridge. From my perspective, it seems that physical damage/misalignment cannot be the problem.
Another update: I seem to have discovered a potential infection on my computer. This was found because Malwarebytes Anti-malware would not start up at all. After trying to update it, loads of external and internal errors poped up. Installation going back to Windows 7 is a potential reconsideration.