robbiesmechanic, I don't know the Ultra LX1, but I'm guessing it's like most other sort of buggy. Sounds to me like you may have a stripped out throttle servo horn insert.

Most servo horns don't fit directly onto the servo post. There will be a small plastic insert you place inside the horn, that couples with the servo post. There is usually a letter on these inserts that corresponds with the servo maker. I.e: F = Futaba, H = Hitech, etc.

Or upgrade to an aluminium servo horn made to fit the servo brand.

Give that a try and let us know.