There's few threads with similar issues floating around ubuntuforums.org. Have you tried using Option "UseEDID" "0" in your /etc/X11/xorg.conf ?
-- nvidia proprietary driver at 800x600 https://launchpad.net/bugs/78057 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
