My DP Ultimate does the same but I believe it is just a characteristic of the manuever itself and not the planes. I believe this because after I learned how to do it I rarely ever experienced the rocking motion afterwards. Remember that the ailerons are EXTREMELY sensetive in the harrier and elevator and try not to use them if possible. You want them to remain as still as possible. Some planes will experience a slight drop in 1 wingtip. If that happens, give a slight amount of opposite aileron and hold it. Steer with slight rudder and slight bursts of throttle and you should be o.k. It will take a few times to get used to it but once you do, you'll have all eyes on you!