Klonky, does he use kwin from 1.4.x branch? It only happens after they switched to fresh branch code and reenabled effects. And it seems only to happen on one 6600gt and 7600gt - so might be really 6xxx and 7xxx related.
A user using same distro and updates (settings too) confirmed that there are no problems on 9800GT.
If I add changelog of kwin rpm from opensuse - factory repo:
Thu 04 Sep 2008 02:00:00 PM CEST
- temporarily disable kwin self-check until the problem is found
Wed 03 Sep 2008 02:00:00 PM CEST
- add libsensors4 support
Tue 02 Sep 2008 02:00:00 PM CEST
- switch to the kwin branch for 11.1 (e.g. bnc#418772)
Thu 28 Aug 2008 02:00:00 PM CEST
- update to 4.1.1
* see http://kde.org/announcements/changel...4_1to4_1_1.php
This is mostly due to changes made on Tue 02 Sep 2008. Before was working.