You've probably not got the correct refresh rates put in for your monitor.
You should have a section which looks like:
Code:
Section "Monitor"
Identifier "Monitor0"
VendorName "Monitor Vendor"
ModelName "Unprobed Monitor"
HorizSync 30 - 105
VertRefresh 50 - 150
Gamma 1.35 1.35 1.35
EndSection
The 'HorizSync' and 'VertRefresh' should be changed to suit your monitor. The manual for your display, or a Google search should tell you what these values should be.
Dave