Shifting print picture

#1
Here's a picture of a larger print that I had the layers shift then go back to correct positioning. As you can see there's another spot just above the bottom arrow where it did the same thing. Any help would be appreciated.

Thanks
 

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#2
I have had the notched drive belt hop a tooth on the stepper drive wheel before, but I can't believe it could hop back on the next layer. In addition, the displacement seems too small to match a belt skip.

Have you reviewed the gcode file to confirm the skip isn't there when it is sliced? I know slic3r sometimes does strange stuff that I can't easily explain, but I haven't tried the program you are using. Repetier Host has a nice line by line graphical way to step through the gcode.