Rotten egg smell is usually a sign of high sulpher content in your gas. If you have a hot running vehicle (like most mad men in Bonnies), your Cat gets super heated. The smell is caused by the sulpher burning through. Easy thing to do would switch brands or grades of gas. If the smell still persists after running through a full tank of different gas, you may want to talk to a mechanic who specializes in this crap. You might have to change your exhaust system...or just stock up on air freshener.