Bug#338426: possibly memory-bandwidth issue

[alve]

I got the same problem - flashing horizontal lines when using keyboard 
and only with gnome. Also I have a low-end graphics chip (sis 740 using 
exclusively system ram) which is known to produce the same problem when 
using it in double headed mode and exceeding the available memory 
bandwidth like that.
So there might be something in gnome that issues some bandwidth-hungry 
action each time you press a key. Just my 2c