Comment 2 for bug 868684

There is no evidence in the logs that there was a GPU lockup event (the prototypical X freeze with -intel graphics). Just the vblank counter failure errors in the log. I assume those messages correspond to when the system became unresponsive?

You indicated this happens multiple times per day under certain circumstances. Do you know a specific activity that tends to trigger it more often than others?

Looking at your dpkg.log file, it doesn't look like any X packages upgraded recently, but perhaps some other package changed and is triggering it. Most of the packages appear to have received only minor version bumps, but I see unity moved from 4.20 to 4.22. Are you able to reproduce this bug with unity-2d (or gnome classic desktop)? If not, you might try seeing if downgrading to an earlier unity version also prevents the issue (apt-get install unity=4.20.0-0ubuntu2 perhaps)

Other than that, do you happen to know which day you first noticed these lockups? That would help narrow down what package might be at fault.