The symptoms of this issue seem to be the same as those in Bug #698121.
I found the same error output in my Xorg.1.log file:
Fatal server error:
[250852.257] xf86OpenConsole: VT_WAITACTIVE failed: Interrupted system call
Bug #698121 is indicated as affecting the xserver-xorg-video-ati
package. I do have this package installed, even though I have an nVidia
card and I have not seen any mention of ATI hardware or drivers in this
bug. Maybe the problem is more general than was previously thought?
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https://bugs.launchpad.net/bugs/742517
Title:
[sandy bridge] xorg freezes with 100% cpu usage
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