Definetly sounds like an out of trim rudder to me. As was suggested before try bigger loops to be sure it's not stall induced. A lot of people trim their new planes with ailerons only. If your ailerons are set with a lot of travel, be sure your not inducing corkscrew with their movement. In the following, do all loops into the wind. Get your plane up on a fairly calm day, after each loop adjust the rudder in the opposite way it corkscrews, then re-adjust aileron for straight and level flight. When it loops straight, it is properly trimmed.