Run
xorgconfig as root in a terminal, this will allow you to configure your display hardware. One of the sections will have an option for the resolutions you want to use, be sure to select all the ones you want (and which are supported).
Else, post the contents of the file /etc/X11/xorg.conf
BTW, to run xorgconfig, open a terminal (xterm/konsole etc)
Code:
$ su - root
<give root password>
# xorgconfig