The problem may be a buildup of carbon or fuel varnish deposits in the idle speed control valve (also called the idle air control valve or IAC valve). The cure for this condition is to clean the valve with some aerosol throttle cleaner or engine top cleaner. disconnect the air ductwork from the throttle body. Start the car and run at about 1500 rpms spray the cleaner directly into the throat of the throttle body. Turn off the engine and let sit for a few minutes. Then repeat the process a couple of times. if that does not work, you can remove the IAC and clean it by spraying into the two holes on the underside of the valve. If not, then you need to replace the valve.If that does not work , then check for a code. the throttle position sensor may be the problem. you can have it scanned at an AutoZone or other parts store.