The servo infront of the main motor will do that. Sometimes they are just aligning themselves to center where the Tx tells them they should be. Something could also be up with the servo if it's moving out of sync with the other ones. I dunno about as far as replacing it... maybe, maybe not. Try plugging a different servo in the same spot and see if it does it. If it does not, then likely it needs to be replaced.

The paper clip mod by choppers rule =) That's what you want to use instead of that sheer pin (I'm guessing that is what you are talking about... the one that goes into the head?)