This gonna sounds nuts, but could be the problem. The tire might be out of round. My buddy had a wheel balanced 4 times, and it still shook like a biotch. Yes, it was balanced, but the tire being out of round was making it shake. Jack up the front of the car, start it up, and SLOWLY let out the clutch so just the one side starts to spin. (I'm not to blame if you get hurt, or kill someone!) I've done it quite a few times just to check things with no problems. Anyway, with the wheel spinning, you would be able to see any motion up and down, or side to side.