Comment 9 for bug 33617

This appears to be a hardware bug. See http://www.thinkwiki.org/wiki/Problem_with_video_related_system_lockup and http://mailman.linux-thinkpad.org/pipermail/linux-thinkpad/2006-March/032672.html

Adding

Option "BusType" "PCI"
Option "DmaMode" "None"

to the "Device" section in xorg.conf fixed the issue for me.