I used to use JR 2721's I believe on the flaps on king cats, had quite a few go flaky on me. The 4131 was a good servo but kind of old now and if I am not mistaken had a plastic gear train. I would personally stick with digitals on control surfaces.

I also just inspect the servos, my BVM Phantom is 11 years old and still has the original servos in it with over 1000 flights. I have replaced a few extensions. I am going back through it this winter replacing fuel lines and airlines if needed. I'll pull out servos and look at them as well while I'm in it, but I suspect I will put them back in.

DR