"I finally got my 2nd 8800 GTS card into my PC and guess what? I think it's malfunctioning...When I was playing World of Warcraft, I had to disable SLI because of flickering graphics. I installed the new nVidia drivers, re-enabled SLI and then websites started to get pixelated. I removed the card and just used the one eVGA sent me to replace my broken 7800 GTX, and it worked perfectly."
I don't know your level of computer knowledge, so don't take offense to this if you already know, but there are a couple different versions of the GTS as well as different stepping versions. If they aren't the same, it could cause problems.
What is your mother board and PSU?