Often times that can also be a slightly high wingloading, what's your flying weight on it? In addition you want to be at a high enough angle of attack in the harrier that you are well beyond the critical angle of attack, ie the angle of attack at which the aircraft will stall. . . if you are just a little bit beyond it then you may see the wings wobble from starting to transition back and forth between stalled and flying. . .try a little more throw/stick input, and a higher AoA.