HI
DO YOU know the incidence of the wing with regard to the stab-a positive wing incidence might cause your problem( you will often be carring some down elevator when you are trimmed for flight -if you are carring positive incidence in the wing ) will the plane track hand off at all air speeds ? if she climbes or dives at different speeds incidence may be your issue or you can click in some down trim on you base approachjust a few clicksshould prevent the porpusing effect you are having-which makes it difficult to make a great controlled landing-do remove the trim when you fly the plane before take off if you choose to do that-TONY