I have had the same problem when my rudder control horn linkage isn't on the hinge line.
I personally think that a lot depends on where the control horns are located on the rudder.
The servo has the screw as it's pivit point it is in a fixed location.
The rudder control horns need to be directly over or very slightly aft of the hinge line. Any further back throws things out of wack because the pivit point is in constant change. It is moving slightly to the left or right each time the rudder is moved.
Try e-mailing RC report (Gordon Banks). He may be able to shed light on it and if he can't, someone on his staff will have ideas too.