Version:           unspecified (using KDE 4.6.1) 
OS:                Linux

Hello.
When I activate the desktop Cube and fool around with it, the PC somethimes(!) freezes. I can still move the mouse cursor, youtube videos play music, but nothing else happens. I can reproduce this crash.

And, sometimes, when I play Minecraft, the same crash happens. Somethimes there are black horizontal stripes on the screen. (300-400px wide, 0.1 - 0.2 seconds). This does NOT happen if I use Fluxbox, FVWM, GNOME, ...

Reproducible: Sometimes

Steps to Reproduce:
Just activate the Desktop Cube by moving the mouse cursor to the upper right screen corner, and then rotate this cube several times and choose one workspace! This does not always happen.

Actual Results:  
The computer freezes, but I can still move the mouse cursor. Making the PC restart by pressing the reset button is the only solution.

Expected Results:  
Everything should just work fine. I expect flawless animations, my PC should do that without problems.

AMD Phenom II X4 955, 8GB RAM, Cougar CMX 550W, Nvidia GTX260, two screens, 1920x1080 each, ASRock Mainboard, ArchLinux x64.

First I thought of a hardware failure, but gaming (Windows) runs nicely. No other WM crashes. It could also be the driver's fault, I'm not sure. I use proprietary Nvidia drivers.

Thanks in advance!