The aux air valve may not be opening fully when the engine is cold. When the engine is cold, you should be able to pull the bypass hose loose from the top of it and with a flashlight, look down inside of it and see if it is fully open. (The engine DOESN'T need to be running.) Some of the cars had a little plastic valve placed in-line in either the hose upstream of the aux valve, or the downstream hose, with a knob to adjust the amount of air that is allowed to bypass the throttle plate when the aux valve is open. It may need to be opened (openned??) a little more or cleaned out inside.