1999 Kia Sephia smoking at certain times

My Sephia will smoke from the tailpipe under certain circumstances. If I am just driving locally or at low speeds it is fine. However, if I have been driving on the highway for a period of time (30 minutes or more) and then come to a stop sign or stop light, it will smoke when I take off, both after shifting and pressing the gas in first gear and second usually as well. If I have another stop soon after, however, it is fine. It is like it is burning off some type of build-up. Do you know what might be causing this? Thanks.