Hello Kevin,
You have already told me that your engine can turn an APC 17x6 at 9000RPM (51mph speed). At 3/4 throttle, your plane would fly at 38.25 mph speed .
From your picture, the rudder deflection seems to be around 40 degres from the center?
If we put those number in servo calc software we have:
Rudder
Average control surface chord (cm) 23
Average control surface length (cm) 36
Optional: Maximum available servo torque (oz-in) 155
Maximum deflection of servo arm from center (degrees) 42
Maximum deflection of control surface from center (degrees) 40


Maximum required torque at maximum airspeed (oz-in) 155

That rudder is so huge that we would need 2 servos in parallele to maintain 45 degres deflection at full speed. Amazing!
I think your servo is not enough strong to maintain its position at 3/4 throttle.[8D]
Scou