Now this is interesting - Scientia reports that doing
actually helped him get over much the same issue. With Hardy's new x server, this doesn't work like it did in the past allowing you to manually specify your card info, but it seemed to help.
sudo dpkg-reconfigure xserver-xorg
It does write a new xorg.conf, so be sure to backup your existing custom-modified one first.
Maybe try that xorg reconfiguration, and apply your mods again.
sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.bak
If your /etc/apt/sources.list file looks like it points to ports.ubuntu.com, perhaps update your software
I can't be sure that Kubuntu for PPC doesn't have issues, so perhaps try Ubuntu as a reference installation, then if that works and you want Kubuntu, you can later install it by installing kubuntu-desktop, and switching to it at the next login.
sudo apt-get update
sudo apt-get upgrade
I know I'm throwing out a lot of "perhaps" scenarios.