Check engine light and bad gas mileage

I have a 91 Accord. The check engine light comes on when I'm going about 65 mph(it doesn't do it every time). After I turn the car off, and restart it, the light does not come back on. Is this something to be concerned about? Also, my gas mileage has dropped from about 23 mpg to about 17. My car has 177,000 miles. I've replaced the air filter. Any suggestions? thanks, Mark