Gnome shell freezes sometimes?

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http://ubuntuforums.org – Hello All Installed and updated i386 Precise today on a recycled HP xw6200 PC with quad xeon and nvidia GT520 graphics, restricted drivers as installed from Jockey. New gnome-shell (3.3.90-0ubuntu1) avoids earlier search bug, but has frozen a couple of times, last one when scrolling in a small LibreOffice document. Unity not freezing. When frozen, no response to mouse or keyboard, can't move windows &c. Can use ctrl-alt-1 to get a tty and sudo reboot from that. Any pointers as to logs I could load from the tty or on rebooting that would have any useful information as to what is ha (Hardware)