proudwinner,
I don't know for sure if it's an aero issue or weight or what, but I know that the Associated 1/12 Jag body just plain doesn't get the job done. Their Nissan and TOJ bodies both work fine, but I believe there are still better choices to be found.
If you want the best handling bodies, the best two I know of are the Protoform Nissan P35 (lightweight version) or the Parma Speed8 (also lightweight version). Yokomo's new 1/12 scale body looks pretty good too, but I haven't had the chance to try one yet.
Trips