My sister had a four cylinder Grand Am and the car actually impressed me with the way it drove (well, except for the massive engine shuddering when sitting still in drive; putting it in neutral fixed this).
Probably a vacuum leak. When I was messin with my PCV valve and the little boot started to split, it idled like crap in drive and was fine in neutral. Fixed that little vacuum leak for like $1.97 and it idles perfectly and shifts better than ever.