Exactly how old is this plane and the servos inside? I had a problem similar once with a plane I built in the early 90s and my aileron extensions had a noise filter device built in which was basically a 7400 series buffer, which happens to run off of 4.8v (5v). When I upgrade my radio and switched to 6v batteries the servos would not work at all with 6v. Removed the filter and all ok.
This may not be your problem.