I have this same problem I'm trying to work out myself... The CEL came on for the first time and I could tell that, at idle, now she was rough and unstable... Hmm? So I get home and pull the ECU back up fuse and let her set awhile and reset everything... Put the fuse back in, let her idle for a minute, and go for a casual drive and let her relearn...
Few days later the CEL comes back on, so I go to Advance Auto and get them to read the code... Comes back P0304, Cylinder 4 Misfire Detected...
So what I'm gonna do first is change spark plugs and see what happens, then I'll follow some of the suggestions on here if this doesn't help out my problem... Car is MY01 with around 70,5xx miles... Hopefully that'll fix her...

IAC should mean Idle Air Controller if I am correct or somewhere along those lines. Sometimes it is gunked up and people clean it out and it fixes it. Other than that it could just be bad and needed to be replaced.