IAC is probably clogged with gunk... you can clean it or buy a new one. I've cleaned three of them so far in each car I bought. Drive it like you want to get somewhere and it will never gum up again. Drive like an old fart, and you'll get real good at the procedure...
2004 Crown Victoria | Sport 2006 Lincoln LS | 2006 F250 Superduty Powerstroke