Comment 5 for bug 592760

I'm experiencing the exact same behavior. Happening on my home system and at work.

Kubuntu AMD64
KDE 4.5 beta 2
libqt4-core version: 4:4.7.0~beta1+git20100522-0ubuntu1~lucid1~ppa1
Video card is ATI 5870 with proprietary driver downloaded from ati.com

Occurs with or without "enable desktop effects" in "System Settings"