tabbsilverado

Got a high idle problem with my 2002 mustang 3.8liter. When u start it cold it idles up at about 2000 or so and stays there doesn't drop at all until you start driving then when your driving and u start to slow down and push in the clutch it jumps to 2500 to 3000 until u come to a full stop for a couple of seconds and does the same when shifting from gear to gear. I've heard stories of the iac valve but not sure . Please help