I was in the great smokies this weekend. D300 mounted on monopod with 200-400 shooting bears. Had some autofocus issues. Tried in all 3 modes. I could manually focus on bear then it would spot focus to fine tune but then a moment later it would try to focus and everything would go way out of focus really bad. I switched between modes and none seemed to work well. It was around 8pm so the light was falling. Thoughts... Is it the camera? Lens? Light?