Possible ubuntu video driver bug

[Derek Trotter]

I have an NVIDIA Geforce 8400 GS card that plugs into a PCI slot.  I 
have no problems with it in XP, but when I installed Kubuntu 11.10 I 
found a problem.  Occasionally the screen freezes.  I can't switch to a 
console.  Also I can't kill the x server.  I know the machine has not 
completely locked up because when I have audio playing, it keeps playing 
even though the screen has frozen.  None of the usual means to shut down 
a machine work.  The only thing I can do is hit the reset button, press 
the power switch for several seconds until the machine shuts off, or 
pull the plug.  I've encountered this on Ubuntu 11.10, Kubuntu 11.10, 
Mepis 11 and Fedora 15.  I installed the LTS verson of Kubuntu 10 and 
haven't had any problems with it.  Does anyone have any idea what's 
going on or how to fix it?

Thanks