I would first check to see if the splines in the servo horn are okay. Sounds like they might be rounded off enough to just slip a tooth with resistance. It could also be that the horn socket has stretched or is the wrong horn for the hitec splines.

If that doesn't get it, I would start looking into having your servo programmed for deadband, etc. If it's not the horn, I would guess it is not detecting center position properly. I do not believe it is your receiver.