Crossfire screen resolution problem.. need help

When i open Crossfire i can only see half of the game (seems like its running 800x600 and my PC resolution is 1920x1080) i went to System.dat and changed first line to 6 and its still not working.. anyone has any clue ?

Does it change it self back to 800, after you change it to 1920?

are you playing full screen? could that be the issue?