windser, i have not run this problem down yet, while it seems like this could be a clogged exhaust, it is not true in my case, unfortunatley i do not have an exhaust right now, i have "used" my extras, you can try
www.bmwwholesale.net these guys have been great to me.......a used cat is a little harder to find, since it is illegal to sell used cats, good luck.