i just format my pc and i installed all drivers and bios things ..
so i wanted to play bf3 with my gigabyte gtx 560 ti but when i enter the game all the screen flashes !!
but im able to play with 1 card what i should do ?
You should provide more details. You say you're able to play with 1 card, yet you say it crashes. Do you have an SLI setup? If so, did you use the SLI bridge and is it enabled in the NVidia Control Panel?
Flickering like the light's going on and off? Or stuttering? The clearer you can provide more details, the more we can help you. Punctuation and grammar actually help to convey your issues more clearly, btw.
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.