The great part of this is that Rodney doesn't seem worried about the traditionally more difficult parts of flying: the takeoff and the landing. (Although I think the Atlantis autopilot does some of that for him.) I suspect what Rodney does is drift off course because he forgets to keep an eye on the heading and make constant, small, course corrections because he has trouble splitting his attention between what's going on around him and flying the 'jumper. And then, when he realizes that he's off-course, he over-corrects. He's not incompetent since John lets him fly a jumper back to Atlantis as early as Childhood's End...he's just not skilled.