So if I understand correctly it has a rough idle but smoothes out once you give it some gas? If so it sounds like the idle air control valve (IAC for short) is sticky or failing. The IAC is a little device mounted horizontally on the front side of the throttle body with two bolts and is used to regulate the flow of air into the motor when it is idling as the butterfly in the throttle body is completely closed. The pintle inside the device should be able to freely move back and forth as commanded by the PCM to regulate idle speed. As the vehicle ages they get sticky or fail completely and the car either idles rough or won't run at all. It's about a $40 part and takes about 10 minutes to change.
