RE: Servo Replacement??????????
If that's the Phoenix 300S Extra 60/90 size, the manufacturer suggests standard servos. You have two servos back there.. which total quite a bit of torque for each side with standard servos. Absolutely no need to use expensive digitals on this plane.
You could upgrade to the next level servo, for perhaps an additional 10 or so bucks, but clearly, not necessary. And as Ken said... that linkage can be a problem. Look at the top of the serov where the servo arm and the clevis meet. If there is black looking crap on there, it could be worn, so check it closely.
CGr