I'll bet it's something to do with the fact that the servo is basically encased in what amounts to be a metal box. Your probably get some sort of electrical noise feeding back. Does it do it only when you are driving the model? Does it work fine if the model is stopped?
Check this thread out - I found it very interesting. I've never had servos glitch, but if I do, I might try this.
http://www.rctruckandconstruction.co...ght=capacitors