I created an account just to help others with this issue.
My story: I have two Geforce 560Ti cards and after installing the 2nd card to run SLI I would get lots of "screen flickering" when starting to play a game. The flickering even affected the opening menus of the game. This was not artifacting or tearing. This was total screen flickers of black and possibly other colors (this is explained later).
I have read all the threads I could find, much like you all have. People have blamed the card itself, the drivers, the graphics settings like vsync and resolution. The issue is with none of that.
THE PROBLEM IS THE SLI BRIDGE.
You have heard that before though. Lots of people have ordered new bridges to try and fix the issue. I have discovered that the issue is not that the SLI bridge is "broken".
THE PROBLEM IS THAT THE SLI BRIDGE HAS A POOR CONNECTION.
Now the fix:
Leave your computer on and go into one of your games. You want to have your screen flickering. Now carefully begin to adjust the SLI bridge connection. I don't have an exact method for this, but I just started with one slide and began slowly pulling it out bit by bit to see how the picture changed. I say pull out because most of us probably installed the bridge by pushing it all the way down, which would make sense. After adjusting how the bridge was seated on my graphics card I eventually (less than 2 minutes) was able to have my screen be flicker free.