Wing rock is usually caused by the wing lifting and stalling in a rapid manner. The wing stalls and drops, picks up speed on the drop, lifts, and stalls again. The stalled wing drops behind the wing still flying, causing the flying wing to accelerate. The whole thing causes the plane to yaw. As you correct the stalled wing, you then stall the wing that was flying. That sets in motion the series of rapid stalls and recoveries that is wing rock. For a harrier you want the wing completely stalled, just as in an elevator. Move the cg back or increase throw in the elevator.

Curtis