richir rich

please help, ive a 96wrx import was running fine apart from slight hesitation between 4-5 thou revs until i put in decat down pipe, plus centre both 3" going into a 2.5" prodrive rear,was pulling well through the revs for about a 100 miles then complete sputter through out the hole rev range,but still idled well ,then after 30 miles of that back to smooth revs,not driven it since.
any sugestions to whats going on