"corkscrewing" out of a loop is usually caused by too much elevator throw. If you are holding the elevator all the way back, and there is too much throw in the surface, the plane will snap out of the loop--causing the "corkscrew" you are talking about. Also, if not elevator, it can be caused by lateral unbalance. Most probably, though, it is due to too much elevator. Next time up, try only going about 3/4 with the stick and see if you get a nice round loop. If you do, then you've solved the problem--back the throw off a little and problem resolved!