service engine light

We have a '99 Chevy S-10. We drove from Columbus,Ohio to Maryland over the weekend. After about 4 hours of driving, the service light came on. We located a Chevy shop on our way. The mechanic, who didn't want to be bothered with it, told us that everything was fine, just as long as the engine sounds fine...
After a few hours the light went out, without any apparent reason. A few days later when we came back from Maryland to Ohio, the light came back on again after +/- 4 hours of driving. Again the light went out after a few hours...
Somebody has any ideas what is going with this ?
Thank you

You need to get the code pulled to have any idea what it is. Even though the light isn't on anymore the ECM will have the code stored. See a better mechanic.