Also... One more place to look. Is the shift happening at about the same height all the time on your prints? If so, I had a similar problem a couple of months ago. One of my Z-Axis pieces had its embedded nut come loose. (Probably from one of the numerous nozzle crashes I had working on some firmware) At certain heights, one side would stick and at other heights it would move too much. That is fixed now, but it can be frustrating as can be when you don't understand what is causing it.