Low Idle Problem

I start my car first thing in the morning and It starts right up and idles fine. But then after the car is warmed up and run for awhile and I shut it off and then start it back up again its idles up and then imidiatly drops to 200,300,400 rpm and somtimes I stall. I rev it and its really rough but then Its fine. Then when I stop at red lights and stuff its fine. I dont get it.