I had the same prob. with a servo and found out after I opened it up, a solder joint had come lose inside. Resoldered it and it is as good as new. I was ready to buy a new $100.+ servo till then. I always open up an electric part after determining it's bad because I've already decided I need a new one and this one is bad, so I cant screw it up any worse. 9 out of 10 times it's just something simple like a bad wire or solder joint and I've saved a ton of money. With the motor vibration and punishment we put these through, it happens alot.