The idle air control valve tends to get gummed up over time. The deposites get hard when it's cold and make it difficult for the valve to move. When warmed up the valves can then function porperly. Try taking the IAC valve out and cleaning it thoroughly with brake cleaner. The valve should flop around very easily in the housing. You can also check the operation by hooking it up to the battery. Positive to the center pin. Ground the outer pins one at a time and the valve should quickly switch positions.