Comment 2 for bug 432980

Not enough information has been provided to troubleshoot this bug. It is generally required to provide a full backtrace for crash bugs.

Offhand, this sounds a lot like bug #368135. You could try turning on SendCoreEvents as described in that bug report to confirm this. If so, this should be fixed with the new xserver we'll be shipping in Karmic. If not, feel free to reopen but please provide a full backtrace. Your Xorg.0.log would be helpful too.