Tyler, Shooting in the dark here with some suggestions. Are you sure the X set screw is set into one of the flat portions of the stepper shaft and not the round? Is there any binding when you move the X carriage by hand? Is it possible that the filament or wire harness got caught on something at that layer? Kind of a coincidence that the shift happened exactly when the layers changed to that smaller area. The only other thing I can think of is the stepper driver strength pot setting. Can you try moving the X side to side with the Repetier (or Mattercontrol) manual control while putting a small resistance on the carriage with your hand. Look for skipping. If it is skipping easily then look at the driver power setting. There is a link on the forums that I'll look for. Nice print!