This is mostly due to a faulty setting for the refresh values of your monitor
Run in a terminal
and click cancel in the box that pops up
In the terminal you now have the horizontal and vertical refresh rates of your monitor
Use these in xorg.conf instead of what's already there
These are in Section "Monitor"
Post back and tell us the outcome
Don't fix it if it ain't broken, don't break it if you can't fix it