Are you running the 64bit version of Kubuntu?
I never could iron down a solution. I did see reference that there was a known bug in the 64 bit version of the ATI driver, and have been waiting for an update from ATI. Your Nvidia experience kind of points it back at Xorg. And I also saw some reference where the KDE folks say it's an Xorg bug as even if the KDE code is bad, Xorg shouldn't allow it to hose the system. Anyway, I did notice that with XGL/compiz the bug went away. But I've taken to running Gnome which doesn't suffer from the bug. Beryl was never very stable for me. There have been a lot of updates to Xorg and KDE, but I haven't gone back to test KDE. I'll need to try it this weekend.
As a note, I experienced the same problem with openSUSE 10.1 and Xorg 6.9 as well as openSUSE 10.2 and Xorg 7.2. It seemed to have started when KDE went to 3.5.5. And I'd guess something to do with the window manager of KDE and it's interaction with Xorg.
I have come to really appreciate running Gnome though. The only bug I had experienced with Gnome is that after a few days Nautilus would spike hogging one of my CPUs, which was a known bug. I'd just kill it and it would come back like explorer in Windows. But since the last updates I haven't seen the bug, so perhaps they fixed it. I think I should have installed the 32 bit version of the OS,