It shouldn't bounce up and down though. It should stay at a constant RPM while warming up. If you replaced your IAC and it's still doing it, it might be a vacuum leak that you can't see. or the coolant line that runs throught the throttle body might be blocked or have a problem with it. I know when I bypassed the throttle body coolant line completely on my lude, it idled like that until it got warm enough.