If we can't find the solution with what we have available it would be nice to see the same design built with the stab in a different vertical location and see if there is any improvement in handling during the same flying conditions. Adding a temporary additional horizontal surface higher on the vertical stabilizer to work in unison with the original stab would probably not be sufficient to make a solid conclusion because we would not 'eliminate' the original 'suspected problem' from the experiment.

I am curious to know if there is any noteable difference with the ailerons raised equally on each side. I believe you stated that you are prepared to experiment with that during your next trip to the flying field. I would hate to see this issue go unresolved because then I won't be able to sleep well at night.