I am not exactly sure, but I think that has to do with the AC. I think the AC needs some engine airflow for it to work really well. My old car had the same problem. The AC could work while idling, but I noticed that when I started driving it became much colder. It could be your freon is so low that it needs the airflow to produce any coldness at all. I'd check into that.
<span style='font-size:8pt;line-height:100%'><span style='color:blue'>1998 BMW 323is Coupe, Black
18" M3 Hyper Silver Rims
2 12" Rockford Fosgate Subs; Sealed
1.0 Farad Capacitor
800 Watt 2-Way Rockford Fosgate Amplifier</span></span>