You need to edit your
/etc/X11/xorg.conf file. Your sync rates are off. If you scroll through the file you should find a section that looks something like
CODE
Section "Monitor"
Identifier "Monitor0"
VendorName "Monitor Vendor"
ModelName "COMPAQ 1520"
DisplaySize 310 230
HorizSync 30.0 - 63.0
VertRefresh 56.0 - 76.0
Option "dpms"
EndSection
You will need to make sure your horizsync and vertrefresh are set in the correct ranges for your monitor. That should fix your problem.