Check the aileron neutral position. They may be drooping. Maybe not visibly, but aerodynamically. This is assuming incident and thrust are per plans. Even an airplane that was built with slightly incorrect incident can be favorably trimmed out with the aileron's neutral position without changing engine thrust angles or complicated incident rebuilding. Bring each aileron up 1/2 clevis turn, fly, repeat until your airplane can be flown through out it's power range without pitch trim changes. I will also add that in my experiences with helping trim airplanes, aileron neutral position has been the most often overlooked issue, and also when it comes to pitch trim problems, the #1 cause of it (when CG is correct). Very often, ailerons are rigged for neutral so that they "look right" with the downward slope of the top of the wing...with little attention being paid to how they line up with the bottom of the wing, and are more than likely drooping slightly. This results in the ailerons acting like flaps...giving the different pitch trim settings with different power settings.