The first time I installed Mandrake 8.0 I had problems with X-windows. When I would view menus in open windows it would leave trash on the screen. It would crash and other things. Basically to fix it, I reinstalled and stopped using KDE. I have had no problems with crashes or trash on the screen with Gnome. I think maybe there is a small issue with the version of KDE that came with that distro.
(Beware: my opinion, I have no proof that this is the case, only personal experience)