Any flex in the substandard push-rods used in arf's can cause flutter. I believe 4-40 should be used on any 3d plane .40 or bigger. Also, it is too tempting to use the inside holes on control horns to achieve max control throw. For example on a 45 oz. servo, the control rod is on the furthest hole out, and on the contol surface, it is on the middle holes, you have just dropped your leverage at the control surface to 22 oz. What could have been perfectly fine in a non-3d environment is now completely inadequate. Aussiesteve suggested reducing the force on the control surfaces by reducing the prop pitch. That is really good advice for any 3d plane.