Im not sure about this, but i do have rudder mixing in with it, wouldnt that be a helping factor, or a accident waiting to happen.
I notice, you say dual rates, which I do fly this plane at 100% because its never affected me till now, so lets say we add dual rates to the picture, now lets explore more, and I really wonder how close the model agwagon is to the real agwagon.

Ive noticed my agwagons tail goes back and forth in flight, mainly due to the amount of dihedral, its tryign to find the right spot, so it wiggles, Ive heard the real agwagon has the same deal going on. its kinda neat I think