black strokes when watching videos

[pellepelser]

hi,

when i watch movies or open a programm with "fast moving elements" (eg. the gimp progress bar when loading) i have black strokes mostly on the left side of the window.

example http://marvin.pellepelster.de/stuff/strokes.jpg

these strokes also occur (but less frequent) when normally using my kde desktop, with x11 i had no problem whatsoever.

my config:

graphic card: ati rago 128 (tried the ati and the r128 driver)
kernel: 2.6.9-r9 (2.6.10 testes, same effect)
xorg: 6.8.0-r4
device section from xorg.conf: