Flying stabs should be statically balanced to keep pitch divergence at bay. Also if the fulcrum is chosen poorly, there may be too much aerodynamic imbalance to overcome even with today's excellent high tork digital servos.
The AC of wing/stab is by definition 25%MAC. Place the fulcrum on or very near the AC projection to the fuse, statically balance the thing and it should work fine
Matt, I just saw your reply despite my daily visiting here in the classic pattern forum. Thanks for the excellent info.