OK, some thoughts....The first thing I would try is an inverted harrier. If your wings do not rock in an inverted harrier, then I suspect that the wing incidence is off a bit. It sounds like the wing is effecting the prop blast over the V-tail at high alpha. When performing an upright harrier, the V-tail isn't getting enough airflow. Of course, if your wing incidence gets moved more negative, then you will be able to moveyour cg back without changing the tail incidence...hopefully! This should allow more airflow over the lower portion of the rudder (fat portion). From what I have read here, I agree with you that it is a high alpha lateral stability problem. I'll bet you don't have this problem inverted because there is so much V-tail above the thrustline.