I'm an RC Modeller of long standing and in my experience, jittering servos are mainly caused by the wiper of the inbuilt mechanical potentiometer being a little dirty thus varying the resistance slightly in response to ambient vibration. The only cure that I know of is to disassemble the servo and carefully clean the pot' and reassemble. NOT an exercise for the faint hearted. In fact, these days, I don't bother as servos are so cheap. Besides, I cant see the small parts anymore.