Do you get a lot of flames and noise at low speeds mate?
Is it lumpy on idle?
Mine did this because my recirc bov wasn't set up properly (a pipe came off) and making the car run rich. A taxi driver stopped me at the lights and said i was flaming at 30mph! :lol:
Don't know why fitting a new exhaust would cause this to happen if it was ok beforehand tho- could be a coincidence but i'd check it anyway just in case...
Probably just the old 'goose doing it's stuff. 8)