The horizontal stab and lateral balance (side to side) are/were definitely contributing to the problem. It may be the whole problem or may not.

Try centering the ailerons and trimming it out with the rudder. Also, as was mentioned by Montague, check the ailerons for flex and warping as well as the fixed and moveable part of the horizontal stab. If any of those are flexing that will also cause the problem.

Flexing is fairly easy to determine in the air. Do a loop at a relatively slow speed (just fast enough to get through the loop), then do a loop at HIGH SPEED (wide open). If the plane does some "unusual" gyrations and/or does not track straight, especially when doing a HIGH SPEED loop, you most likely have something flexing (most likely the vertical or horizontal stab). Make sure you are AT LEAST "3 mistakes" high, especially before doing a high speed loop. I would suggest A MINIMUM of 150' up AND be ready to chop the power and correct the plane, IN THAT ORDER, if "unusual" gyrations occur.