I just started playing Skyrim again and my screen flashes quite a bit when I'm outside, and not when I am in dungeons or houses. I have been checking around the forums for the same problem but I have only found stuff like shadow flickering and such. I am running 2x 6850's and a i5 2500k.
Edit- The flashing is like a dark green flashing.
More aboutskyrim screen flashing help
Do you get the same flashing when you Shout? Try Shouting indoors too.
Turn caps lock off, that solves everything, if for some supernatural reason it doesn't work, have a look at your refresh rate, or in the graphics settings turn on V-sync and turn off any filtering effects.