It sounds like either the refresh rate or dimensions of your X server are too much for your monitor to handle. If you can find your config file (either /etc/X11/XF86Config or /etc/X11/xorg.conf) and post it here, people can check to see what your settings are and recommend a fix.
Even if I'm not right with that guess, it
will be some kind of config problem and tweaking your config file
will fix it!