Service enging light

Make: Chevrolet
Model: C1500
Year: 1993
Mileage: 213000
Engine: 5.7L

Question: My "service engine light" comes on only when I drive for about 20 min. Or so. (Either at a consistant 60 mph on highway or stop and go in the city) The truck runs fine and does not feel that it is missing. When I turn off the truck, and turn it back on, the light will be off, until I drive it for that long again. If possible, can you tell me what a probable cause of this may be? I'm sure I'll get the code read here pretty soon. Thanks.