One of my Fedora 10 systems used to freeze very frequently. After lot of looking around I found its because of "KWin Composing" which gives OpenGL driven special effects for desktop. Unfortunately, Linux has always been bad at radeon drivers, so it better to disable these effects especially if you have radeon video cards.
in [Compositing] section change
Enabled=true to Enabled=false.
Reboot after this change. Now I never get any system freeze - as is expected from solid Linux system :)