The other name for this phenomenon is an accelerated stall. With too much elevator you are raising the wings' anlge of attack so fast that the airflow separates from the upper surface. And as the other poste mentioned you may have a warp or other mis alignment because it goes the same way each time. I had a friend with a HOB FW-190 beautiful airplane, would do a perfect 1-1/2 snap to the up right at the top of every full elevator loop.