That looks like an xorg bug. Could you attach /var/log/messages
/var/log/syslog /var/log/Xorg.0.log to the bug after getting the crash?
** Changed in: gnome-screensaver (Ubuntu)
Importance: Undecided => Medium
Status: Unconfirmed => Needs Info
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Screensaver causes system lockup, or freeze
https://bugs.launchpad.net/bugs/94449
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