Older analog servos don't always work right with the modern radios. The way the input data is streamed to them on newer high-speed 2.4 equipment makes them often try to move/twitch/"grind" constantly even if you can't see it. If you can change the "framerate" or response speed on that radio, try a slower one and see if that helps. I had an Airtronics 737 that was the perfect 1/10th pan car servo but it was also left behind by the digital radio revolution.