Most xorg problems can be solved by reconfiguring the server, first off though make a backup of your current xorg.conf, open a terminal and enter:

sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.bak

If you need to return to the previous configuration you can then type:

sudo cp /etc/X11/xorg.conf.bak /etc/X11/xorg.conf

Then you can:

sudo /etc/init.d/gdm stop
sudo dpkg-reconfigure xserver-xorg
sudo /etc/init.d/gdm start

If you still find that you are unable to get the resolution and refresh rate you are expecting then you can manually enter the horizontal and vertical refresh rates. This information can be obtained from the manufacturer of your monitor. Open xorg.conf in a text editor:

gksudo /etc/X11/xorg.conf

Go to the monitor section and enter the following lines, with your own values of course:

HorizSync 	31.5 - 48.5
VertRefresh	50.0 - 90.0

Then save and restart x as before.