I have a 4.6L V8 SOHC MOTOR w/101K miles on it. The car is now running rough @ idle 400-900 rpms @ idle. I have a Steeda tune/chip from 1999 when I first picked up the car which adjusted the shift points on my automatic & also adjusted the speedometer as I changed the rear end gears to 3:73s. I have no catalytic converters & put in MIL eliminators to trick the computer & put in long tube headers an H-pipe & Flowmaster mufflers. Car has run great & smooth w/this setup for over 13 years. I replaced the original platinum spark plugs with Motorcraft platinum plugs, also changed fuel filter, PCV valve and the IAC valve & the car still idling & taking off rough. What can be the problem...wished I still lived near Steeda, don't know any good Mustang mechanics in Brooklyn, NY.