My '97's CEL came on a couple of years ago, and was on steady for about 6 months, I just ignored it, because all was normal.
Then during a trip from here (St Louis) down to Texarkana, I had her turned loose at about 95 mph for a long stretch, and the light went out.
It stayed gone for about 3-4 months, and came back. I tried the same drill, and sure enough, after 5-6 miles at 90+, it went out.
This pattern continued for about 2 years. Then one night someone stole my gas cap. I bought a new one, and the light hasn't come on since. That was back in about March.
Go figure.