Here's some things to try:
1) Do you have Vsync enabled? I've heard this problem can be cause by too high of a framerate. Also, can you run something like Fraps that tells you your FPS while this is happening, so we know if it's low or high?
2) Reset your SLI bridge. Try flipping it around. Just make sure it's snug.
3) Try different SLI options in the NVidia Control Panel. Try both Alternate Frame Rendering 2 and 1.
4) Skim this thread for other ideas: http://www.tomshardware.com/forum/335829-33-flickering