Sometimes it can be indicative of a serious problem, but a lot of the time the check engine light comes on when your car has poor fuel efficiency. This sounds like exactly what is happening with your car, as the light turns off when you come out of an idle, thereby increasing your fuel efficiency. I had this same problem a while back with my old Jeep, and I was concerned at first and took it to a mechanic several times. Each time they found nothing wrong, and on the last visit they finally told me that it just had to do with the fuel efficiency. After that, I noticed that when I was driving in the mountains, or idling a lot, the engine light would turn on. But, when I took it out on the highway where it ran better, the light went off. I imagine this is what is happening with your car. If I were you I would get it checked out at least once to make sure, but in all likelihood it’s not a major problem.