Well, I would recheck the geometry, as something is not right. Whether one side of the rudder horn is longer than the other, or the servo is not sitting exactly on the centerline, but, I have yet to see a crossed pull-pull setup work yet that didn't have an offset.

Some further info to digest: [link=http://www.andreyko.com/html/ackerman.html]http://www.andreyko.com/html/ackerman.html[/link]

Good luck,
Spar