Comment 115 for bug 546578

@Stefan: given that you are running an nvidia card, your issue may be related to mine. I've filed a separate bug:
https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers-96/+bug/585685
Have a look at that & see if perhaps it may be caused by your nvidia-173 driver. If so then it's something common between the -96 and -173 drivers.