X server crash with VMWare or VirtualBox + nvdia binary driver + session switch

[John Carlyle-Clarke]

My wife and I both use the same PC, so multiple user sessions is a 
must-have for me and mostly seems to work OK.  However, if I have VMWare 
Server or VMWare Player running in my session and she switches session, 
my X server will crash, kicking her out of her session (which remains 
active) and back to the KDM login on my VT.  She then logs in again, so 
I find later two running sessions for her, and mine vanished.

As I say, this happens with VMWare Server and Player.  I also recently 
discovered that it happens with Virtualbox too.

The crash sometimes happens immediately when the session is switched, 
and sometimes a while after.  Occasionally, it does not happen at all 
(usually when you are trying to test it).

I ran with the open (nv) driver for a while, and couldn't provoke a 
crash, so it's likely that it doesn't suffer from the problem.  Before 
long I got sick of the bad 2D performance and swapped back.

I am pretty certain that it used to happen with fglrx when I had an ATI 
card, but I was new to Linux and that was a while ago.

Has anyone seen this problem, or have a solution to it?

How can I trap errors or get some debug trace out of the X server when 
it goes?  At the moment, nothing useful is written to 
/var/log/Xorg.0.log and others.

Thanks in advance,

    John