1992 Isuzu Trooper check engine lite-shut down

My 92 Trooper check engine lite comes on after driving at high speeds on the highway--even if doing 55 or 60. After 10miles of that the check engine lite comes on and when I come to a stop it shuts down. Sometimes I can get it to come back on and I drive a short while, a couple of times it has had to sit an hour or so before it starts back up. I dont have this problem when driving on the street, even long distances. Different mechanics have checked it out and cant find a problem even after driving (although I doubt they drove a very long distance).