I used to have this happen, and the reason was because of a bad knock sensor. I would start the car, and the CEL would come on, and then after about 10 minutes of driving, it would go away. After that, If I stopped the car, it would be gone, but someitmes flash on for a second or two. After lettign the car sit for an hour or two, the same thing would happen. If you go to this website:
http://svx-iw.com/ and then click on SVXipedia, and then click on "checking error codes", you can do the diagnostic yourself. It is really simple and quick, and will let you know what is causing your CEL to come on.