I have a 88 62 with 170,000 miles. For the past year of so my check engine light has been coming on randomly at highway speeds. Sometimes it happens only 30 minutes into a drive, other times it might come on 2 hours into a drive, sometimes it never comes on at all. After I kill the motor and crank it back up the light goes away. I have no idea why its coming on because the truck runs well and all other gauges look good. Anyone have any ideas?