changing user with new session lock up computer

In opensuse 12.2 with KDE.
If I change user without exiting from actual session and after new login either close new session or try to go back to previous session, everything lock: clicking mouse is useless and ctr+alt+F? or any key combination are useless.
Only hardware reset works.

How much memory do you have?

What video card and driver?

from sysinfo:

memory 3,7 GiB (+ swap 7.1GiB)
Total emory (RAM): 3,7 GiB
Free memory : 1,6 GiB (+ cache 1,1 GiB )
Swap available: 7,6 GiB

Linux 3.4.33-2.24-desktop x86_64
openSUSE 12.2 (x86_64)

video card: nVidia Corporation
Model: GT200 [GeForce 210]
Driver 2D: nouveau
Driver 3D: nouveau Gallium (8.0.4)

Thank you for your help