On top of your manifold there is a device called an Air Regulator. It has an orifice that is open when the engine is cold but closed when the engine warms up. Kind of the EFI answer to a choke. If it is gummed up it will not close and you have too much air at idle. Try taking it off and loosening it with some WD40 and an air hose to dry it out. Often you have to replace it. Put it in the freezer to close it and then the oven to see if it opens.
It doesn't sound like an AFR problem. That would cause either low idle when cold or fast idle when warm.
Check your idle adjust screw on top of the throttle body. It seems that when the Air flow regulator is open (cold) it's getting enough air at idle, but when the AFR heats up and closes there is not enough air to support idle. You possibly adjusted the idle while the car was cold and the AFR was in an open state. Try unscrewing the large screw with the spring on it and see what happens.