Xorg.conf is one way.

Easier to first go to [depending how your set up] a menu editor and
Other then look for "Screens and Graphics" check the box so it is
available in the menu.

Go to:
Applications--Other--Screens and graphics [select your monitor and
resolution]. I have a Samsung monitor on my spare machine and i always
use this method. Ubuntu seem to do one or two odd things from version
to version. The location of the "Screens and Graphics" for one thing -
some hardware seems to get left out in later versions and yet was in
say 7.04.