I don't mean this to be sarcastic, but if you are using the absolute cheapest servo on EBay/Amazon, you can expect performance issues no and then. Granted, I have no idea what kind of servo you bought. But, in my own tinkering, I rarely buy the cheapest item if I have a choice. I buy the next level up and seem to get better reliability and performance. In some things it's true, "you get what you pay for."

Maybe try a more expensive servo and see if it services you better.