Sounds like you have an X freeze rather than a crash. You probably want this:
https://edge.launchpad.net/~ubuntu-x-swat/+archive/x-freeze-test
backtraces don't give much useful info with freezes.
** Changed in: xorg-server (Ubuntu)
Status: Incomplete => Invalid
--
Apport doesn't collect
This is not a problem in apport. If the Xorg.0.log does not have a
trace, then I don't think that it was a real server crash. Could you
attach an Xorg log where this happens?
** Changed in: apport
Status: New => Invalid
** Also affects: xorg-server (Ubuntu)
Importance: Undecided
