Weird display problem

[Daniela]

Hi all!

There's something strange going on with my system. Sometimes, all of a sudden, 
there are lots of colored vertical lines on my monitor. When I switch to a 
terminal at this moment, there's a mix of some funny characters and some of 
the characters that should normally be there. A few of these are only 
displayed half, i.e. every other row erased. The system responds to commands 
as normal, it just looks a bit messy on the screen. The problem doesn't go 
away until I reboot.

I thought it could be faulty hardware, but then I found out that it is 
reproduceable: It always happens when I visit 
http://directory.google.com/Top/World/Deutsch/Wissenschaft/Mathematik/Angewandte_Mathematik/ 
in Mozilla and scroll down the page. I once had this a long time ago with 4.8 
(now I have 4.9), but I can't remember the browser I was using and the exact 
site (it was something in Google directory).

How can I solve the problem, or at least find out what causes it?

Regards,
Daniela