Are you absolutely sure that all 4 four of the XY steppers are connected? When the belts were off did you make sure all four pulleys were moving in the correct direction and none were “stuttering”? This sounds like it could be a symptom of one open wire to one of the steppers. An open wire will cause one side to randomly step back and forth with the other side dragging it along in the right direction.
Reply To: Jagged Diagonals?