possibly idle valve / idle control unit, sometimes you can just clean them out with carb cleaner and they're fine, to be sure though get it plugged in to a diagnostic machine, your car stores all its fault codes then you'll know exactaly what you need to mend it. by the way when you have to rev it in neutral will it rev cleanly past 4000 rpm, usually if any engine sensors are faulty you won't be able to.
brett, October 2011
