I had a similar issue right after I hacksawed my muffler off. The car would idle high as hell (1K at least) at then slooooowly go down to about 500 after warming up. I'm gonna assume you haven't done anything drastic like that, but I figured it was worth mentioning.

 

the iat sensor is the one that plugs into the elbow, not the little blue thingy, but the black piece that has a connector plug on it. when the sensor is pulled, it looks kinda like a large caliber bullet. its only like 10 bucks at auto zone.