[GTP Dad]

If you changed the IAC and the problem continues, remove and clean the throttle body with throttle body cleaner. The area where the IAC allows air into the throttle body may be blocked with carbon and you may not be getting good flow. One other thing to check for is a vacuum leak. Usually this can cause high idle but it can cause rpm fluctuations. I would start with the area around the IAC.

As for the trac light coming on that is due to a low voltage condition due to the engine rpm drop. Once you correct the other issue that should go away.

Finally, have the fuel pressure checked when the car is hot. Sometimes the regulator will malfuntion and cause this issue. That is why you need to pump the gas to start it when hot.