What's the car and fuelling system?
My thoughts here are to look at the throttle housing, butterfly seal to body and operation first along with the Idle control valve and wiring. When you start the car the idle control is actuated and it moves in or out, small movements, to allow the correct air fuel mix to settle the engine at idle. So on start up the engine adopts a faster idle to settle the running and when happy it drops it back to standard idle, 750 + or - 25 rpm.
Disconnect the air ducting to the throttle body and using a hooked piece of wire coat hanger slowly push the top of the butterfly open with a finger enough to slip the wire in between the butterfly and the body. The hooked end stops it disappearing down the intake! a little securing tape doesn't go amiss either. Now start the car, if it starts and runs suspect the idle control valve, they do get dirty so remove and clean. Try not to clean intake items in situ as it is best not to allow solvents into the cat as that could be very expensive!!! choice of cleaning agent is carb cleaner not brake and clutch cleaner. It's expensive but one can will last you a lifetime. Then while you are there clean the throttle butterfly and barrel by spraying cleaner on a rag over a finger. clean both sides of the butterfly and all contact areas of the barrel. Best to disconnect the electrical plug to the throttle actuator first before moving the butterfly for cleaning. The electrical method for the ECM to move the valve will receive an electrical reverse input when manually moved and these will be confusing and random as you clean and can damage the ECM.
Hopefully this will ascertain the problem for you.
