It seems the new ATI drivers fix this. I have not yet tried myself. See in the forums:
http://ubuntuforums.org/showthread.php?t=204910 -- Lockup problems with both the free xorg ATI driver and the proprietary fglrx driver, using various ATI cards https://launchpad.net/bugs/30447 -- kubuntu-bugs mailing list kubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/kubuntu-bugs
