850 check engine light - no trouble code

Folks, Check engine light came on. I have a scanner but no trouble codes show up. Here's the thing: light comes on intermittently, mostly at slow speeds like in the city; on the highway it goes off and stays off. Light comes on at start, after some minutes it goes out, but comes on again when idling or starting from red light. There is no obvious engine problem that I can see or hear or feel when driving. Anything I can look for before going to the dealer? I checked the gas cap.