This is in essence a bug in the graphics driver. In fact I had the same problem earlier, but it was fixed with a compiz workaround. See bug 269509 for details.
Can you please do "glxinfo > /tmp/primary.txt" in your normal working session, then start a guest session, let it settle for a minute or two, and then type blindly:
* Alt-F2
* glxinfo > /tmp/guest.txt
* Enter
* Control-Alt-Backspace to kill the guest session
Then please attach /tmp/primary.txt and /tmp/guest.txt here.
This is in essence a bug in the graphics driver. In fact I had the same problem earlier, but it was fixed with a compiz workaround. See bug 269509 for details.
Can you please do "glxinfo > /tmp/primary.txt" in your normal working session, then start a guest session, let it settle for a minute or two, and then type blindly:
* Alt-F2 Alt-Backspace to kill the guest session
* glxinfo > /tmp/guest.txt
* Enter
* Control-
Then please attach /tmp/primary.txt and /tmp/guest.txt here.