May be a gummed up idle air control valve that sticks when it's cold. Remove the valve and spray the heck out of it with some carb or brake cleaner until it frees up and moves easily. If you feel adventurous you can also remove and clean the throttle body and flame trap device. The vacuum hose connection on the flame trap housing will get plugged over time and need drilling out to clear. Also make sure the air filter is ok and there are no intake system leaks.