Before replacing your IAC or FITV (idle air control valve and Fast idle thermo valve), make sure your not overheating at all. Overheating can melt the wax plungers inside and ruin it, even if it overheated just once. Also make sure you have enough coolant in the motor, air pockets can prevent those valves from closing, so can a plugged coolant line. Also check your idle screw if you have one. Make sure its not all the way open.
Basicaly those 2 valves have coolant flowing through and when it warms up, they close valves that stop air from coming in to lower the idle. Your MAP sensor, TPS and rpm signal are gona start argueing in your pcm causing your iac to be controlled improperly, making idle go up and down every second.