What monitor are you using? Do you have the tech specs of it? Specially the vertical and horizontal refresh rates? If not Google for those.
Once you have them, you' ll need to edit the file '/etc/X11/corg.conf' via sudo, to add those values.
The '/etc/X11/corg.conf' had to end something like this (use the values for your monitor!!!):
And also, its very likely that you'll have to add the desired resolution to the another section of the configuration file as well (use your resolutions!!!):
Identifier "Samsung SyncMaster 920NW"
First of, I would say that you should get the refresh rates, then make a copy the current xorg.conf file, and then edit it.
Identifier "Default Screen"
Monitor "Samsung SyncMaster 920NW"
Device "NV11 [GeForce2 MX/MX 400]"
Option "AddARGBGLXVisuals" "True"
Modes "1440x900" "1280x1024" "1280x960" "1024x768" "800x600"
Viewport 0 0
To make a copy of the current config file, type this on a Terminal:
It will ask your password after the ENTER, type it, and then ENTER again.
sudo cp /etc/X11/corg.conf /etc/X11/corg.conf.working.copy ENTER
To edit the file, if you are using Gnome, type this on a Terminal:
If you execute this right after making the copy, it will not ask your password once again.
sudo gedit /etc/X11/corg.conf ENTER
When you are done editing it, save it. and either reboot the box, or type Ctrl + Alt + Backspace to force a re-init of the graphical environment.