sounds like it is the cat. I had that same problem with my cat in my old car. I also just ripped it right out and replaced it with straight pipe. (I don't understand that 'fire in the exhaust' thing) With the car cold, get under the car, and bang on the cat with something, if it rattles then obviously the comb inside the cat has disintagrated and has broken apart. Either get a new stock cat, a high flow cat, or run straight pipe.