I thought that was done on purpose. I don't think he was supposed to move naturally.
If that's the case then what a huge mistake!!!
But I don't think that's the case as I have seen that kind of hurky-jerky kind of motion in other poorly done CG characters. Plus Stayne wasn't the only character in the movie that looked that way at times, for example: Stayne's horse. When we see Stayne for the first time, he climbs up on his horse, I think his horse rears up and they ride away. Looks atrocious!