David Culter and Rotaryphile are both right. You are snapping out of the loop by trying to loop too tightly and stalling the wing. However rather than limit the elevator travel, learn to use the stick gently. You will be a better pilot if you learn to fly through the loop rather than just slap the stick back to the limit. The beauty of proportional control is that you can move the surface only the amount that is required for a particular maneuver.