mouse cursor freezes on session change

[Miguel Blay]

hello
i have YDL 2.3 on a PowerMac G4 Cube...here is what happens:
when I change session (Gnome to KDE or user to root) my mouse cursor 
freezes and gets stuck on the screen...I wonder what kind of conflict it 
can be...maybe mouse vs keyboard or is it the graphic login 
interface...maybe i should change it to text but then ...I know that 
startx will launch KDE ...what is the command if i want to try Gnome ?

mouse: one click, apple pro optical mouse
keyboard: apple pro (French...azerty)