Unable to switch X sessions between different VT. System freeze and need power down to restart

[LinuxBox at infinito.it]

Hi,
it's my first post into a mail list so I apologise if I'm doing/writing 
something wrong.
Problem: Any attempt to switch between two different X sessions freezes the 
whole system. Power off/on is needed to recover.
I would use the switch user option in my Mandriva 2006 box but any attemp to 
use this function lock the system.
I also used > alt-ctrl-fx > xinit -- :1    to start a new X session getting 
the same result.
The new X session is started and I can use it, but attempting to came back to 
the original X sesstion (usually Vt 7 by alt-ctrl-f7) freezes the whole box.

Here is my system:

AMD Athlon 3500+
Mandriva Powerpack 2006.0
MB Asus A8N5x
Graphic Card: Asus Ati X700 PRO PCI-E

I've just upgraded to the last ATI drivers 
ati-xorg-8.22.5-0.1.20060plf
dkms-ati-8.22.5-0.1.20060plf
but the bug is still there.

Xorg -version gives:

This is a pre-release version of the The X.Org Foundation X11.
It is not supported in any way.
Bugs may be filed in the bugzilla at http://bugs.freedesktop.org/.
Select the "xorg" product for bugs you find in this release.
Before reporting bugs in pre-release versions please check the
latest version in the The X.Org Foundation "monolithic tree" CVS
repository hosted at http://www.freedesktop.org/Software/xorg/
X Window System Version 6.8.99.900 (6.9.0 RC 0)
Release Date: 01 August 2005 + cvs
X Protocol Version 11, Revision 0, Release 6.8.99.900
Build Operating System: Linux 2.6.12-12mdksmp i686 [ELF]
Current Operating System: Linux kowalsky.localdomain 2.6.12-15mdk-i686-up-4GB 
#1 Mon Jan 9 16:13:46 MST 2006 i686
Build Date: 16 September 2005
        Before reporting problems, check http://wiki.X.Org
        to make sure that you have the latest version.
Module Loader present

After an attempt to upgrade my Xorg stuff up to the latest level I've been 
unable to get the X server starting any more. So reistalled the system. 

Any suggestion?

Thanks in advance

Roberto