Find the "Device" section containing the "Video Device" in the file /etc/X11/xorg.conf (you can edit it doing "sudo gedit /etc/X11/xorg.conf") and replace it with this (or just add the Option sections):
Code:
Section "Device"
Identifier "Configured Video Device"
Option "AccelMethod" "EXA"
Option "MigrationHeuristic" "greedy"
Option "NoDDC"
EndSection