I am new to form discussions, my obsvations of what happens at higher speeds is that as speed goes higher control surfaces take on
loads squared by the speed. Seams to me trim problems might be caused by the lack of torque from the servo controling that surface. I
would sugest you take a good look at the geometry of the linkage and the torque of the servo controling it.I have had simular problems
with elevator servos during high speed turns and loops. Digital servos and higher voltage helped my problem. Hope they will help yours.
Alan3dflyr