top part of the Monitor screen flickers

[ruhollah]

I have "flickering" problem with my LCD monitor in buntu 9.10. the top part of the monitor screen flickers horizontally and vertically (mainly horizontally). I did update manager, but it did not help. also I tried to manually edit the xorg.conf and add some vertical and horizontal frequencies, but no improvement (display did crash and I recovered the xorg.conf).
do you have any suggestion?
here are some specs:
my desktop is gnome
monitor: Dell 2407WFP 24"
graphic driver: NVIDIA
Driver Version:185.18.36

xorg.conf content:

Section "Screen"
Identifier "Default Screen"
DefaultDepth 24
EndSection

Section "Module"
Load "glx"
EndSection

Section "Device"
Identifier "Default Device"
Driver "nvidia"
Option "NoLogo" "True"
EndSection