I may be wrong, I always had issues running analog on digital receivers, so it could be an issue, or in his case it could be something else, and its not being affected. I guess it all depends on whats what.
Reading through I just pulled some JR Racing Z270 Servos off another car. They are Analog servos and could be the issue seeing as your running it off a 2.4ghz system. I would get newer servo that was built to run off the 2.4ghz system and try that first!