Get to the console, login as root.
and try this command:
pico -w /etc/X11/XF86Config
if successful it should display your X11 configuration file. Now hit CTRL+W and type this into the prompt: SubSection "Display"
now that should take you down to where X11 has your screen resolutions defined. For me I have this:
Modes "1024x768" "800x600" "640x480"
Now the parts in bold
are your defined resolutions. Remove the first entry, NOT line, but the first entry like for me I have "1024x768"
Now hit CTRL+O and hit enter key to write the file. Now hit CTRL+X to quit Pico. Now either you reboot the machine or just log out of root and login as your normal user and then do: startx
That should do it and if not repeat as needed to remove any high resolutions your card will do but your monitor won't accept. I hope this helps you. :)