It seems that nearly every Hitec servo I've ever bought has a "jittering" problem. With all the controls dead center on the radio, not touching anything, they shake slightly. When you give it throttle or steering input, it's sometimes OK, but many times, it'll jerk heavily in one direction or the other. I've had this happen with no less than 8 metal gear high tech servos, and a few plastic geared servos. I will say that the big daddy digital that's in my 1/5th scale car works flawlessly.
I run primarily high end Airtronics analog servos, and JR digital servos, and have had great luck - none of them have the problems that I've experienced with the Hitecs. I called tech support a few years ago, and was simply told that it was radio interferance, and that I should be able to solve the problem by locating the source of the radio noise. Like I said, nothing else I've used has the same problems.
The latest headache is an HS81-MG that I'm using in one of my electric race boats. Just built the boat, haven't dropped it in the water yet. The servo shakes the second I turn the power on. This is a 21" 12 cell boat, so I'm a little concerned about the eratic servo action.
I ran a KO Precious up until around 3 years ago (probably the best performing radio I'll ever own), and now run an M8. Is there some sort of incompatibility issue here? I don't get it. I'd like to save some money by running Hitec digitals, but I'm afraid to experiment by purchasing one.

Hi,

That does sound strange to have servos acting like that. Have you tried coiling your servo wires around an RF Filter? Those seem to cure a lot of twitching problems with servos.

-Mike