My screen is flickering too, but i found intresting response here

Hi,
Not sure why and how, but it appears that there is a relation between the Wireless network drivers and the display. It sounds weird but as a work around, try and disable the wireless connection on the laptop or if you are using the wireless to connect to the network, try to adjust the wireless card power settings under the wireless NIC configuration (right click and open network connection properties>right click on the wireless connection and select properties>select the wireless NIC>click configure>select the advanced tab>find the power management in the list>set the slider to somewhat lower setting). This seems to help. It might have something to do with power distribution.

I use ubuntu linux and i can't adjust power like that. If anyone knows how to do under ubuntu please tell us.

Link: http://blogs.infosupport.com/blogs/marcelv/archive/2007/09/26/Dell-latitude-D620-display-driver-problems-finally-fixed-with-a-driver-older-then-Vista-itself.aspx