If you have the original IAC motor on a higher mileage car the idle hunting seems to be a common problem. I have found that simply pulling off the IAC motor and hosing it out with cleaner helps it out a lot. My '90 vert that I just got this year had this problem so bad that it would actually stall. I cleaned out the IAC motor and now it idles perfect. I'm sure this isnt a permanent fix and that it needs to be replaced but it works for now.