Weird print issue: X and Y get shifted at random heights

[mattanonymous]

Any idea what causes this? I've had it happen on several large parts. I'm slicing with Slic3r, but the rendering of the g-code looks fine when viewed in pronterface.

Also, toward the end of printing an identical part, the printer just stopped. The nozzle stayed and bed stayed heated and the homing g-code wasn't run (I assume because it didn't make it to the end).