Now keep in mind that I am a long way from an expert on posting; however, I believe that you might be posting from your feet. I mean that you put all of your weight on your feet to push yourself up out of the saddle, that will make everything seem rougher and uncoordinated. Your feet really shouldn't move, you should use the horse's own motion for the upward momentum but keep contact with your knees and thighs. Sorry I am not better at explaining things, I am sure that somebody like JDI or Maura can explain it much better. Also, on your endurance saddle, I think your stirrups might be a hole too short. Ideally, if you stand in the stirrups, you should be able to put a fist between your pelvic bones and the seat of the saddle, but no more.