Mine does that also. However, it does it when I abruptly reduce the throttle from full to idle for landing. I soon determined that after the zoom, the trim was just right for approach speed. So, I just got to idle more slowly or when I drop the throttle quickly I apply some down elevator until it slows down. I too was going to put in a degree or so of upthrust but I would have to go to a lot of trouble if I kept the engined cowled. I just decided to live with it.