What you are refering to is called "wing rock". Your question could best be answered in the 3D forums but I will tell you from my experience that many of the smaller 3D planes will have excesive wing rock in harrier. The wing rock is caused by one wing stalling and falling then you correct and stall the other wing and you just go back and forth. You may be able to tame it by laterally ballancing your model. Sometimes flapperons or spoilerons can help as well.