Have you installed the ATI Proprietary Graphics driver on your machine ?
http://ubuntuguide.org/wiki/Ubuntu_E...iver_.28ATI.29
Start with that, and then ensure that the vert and horix refresh rates stated in /etc/X11/xorg.conf are set to the correct values for your display per the manufacturers specifications.
Code:
Section "Monitor"
Identifier "Dell 2407"
HorizSync 30-76
VertRefresh 50-81
Option "DPMS"
EndSection
On the down side Dell didn't bother to provide the low values for the refresh rates, only the MAX for H & V (76Hz and 81Khz respectively) I hate when they cheap out like that...