Comment 46 for bug 57153

I was also affected by the problem, and since I noticed that the "Monitor" section in /etc/X11/xorg.conf contained just two lines[1] (as it is the case in the xorg.conf file posted above by Sergio Oliveira), I have performed dpkg-reconfigure xserver-xorg several times to no avail. Thanks to the solution posted at ubuntuforums.org I installed manually the corrected package and everything was happily fixed.

Credits go also to Rodrigo for the quick fix!

[1] these:

Section "Monitor"
        Identifier "Generic Monitor"
        Option "DPMS"
EndSection

instead of (currently working):

Section "Monitor"
        Identifier "Generic Monitor"
        Option "DPMS"
        HorizSync 30-86
        VertRefresh 50-160
EndSection