Extreme layer shifting

Hey!Having issues with my Wanhao D5s mini. It's experiencing EXTREME layer shifts (shifting 4-5 inches during a print, in multiple increments of 1 inch every few layers). This shift only happens in x-direction.I have troubleshooted, and come to the following conclussion:The belts are tight enough, it seems like. The pulleys are tight on the axles. However:When moving the motors using the built in "Move X 10mm", it works great and moves smoothly, even when quickly pulling it back and forth across the full distance of the print. However, after about 10 full laps, the motor starts stuttering, and looses it's 0-point by about an inch. The stutter means the extruder moves 1 inch, freezes, moves 1 inch, freezes, over and over. After the full lap, it zero's at about 1-2 inches before it's normal end-stop.What might be causing this?Thanks