Post ur xorg.conf file located under /etc/X11/ directory to us.
If possible truncate the rest and leave the [Display] section only for posting. I believe it's a very long config file.
I suspect is ur graphic confguration is the cause.
Please inform us ur video card and monitor model.
Wrong monitor resolutions and refresh rates can give u such headaches.
It's ok, that's the fun in it, solving a challenging puzzle.