My 1990 F-150 is running quite poorly on the highway. it runs great in town, but when I accelerate on to a highway it begins to start ******* and sputtering. It has a hard time holding highway speeds and getting up to highway speeds from 50mph on up. When I pull off of the highway, it runs very rough in town but eventually "clears up" and begins to run fine again. After pulling off of the highway it sputters and misses as I accelerate. If I shift into neutral and step on the gas it seems to be O.K., but when I put it back in gear it runs rough again until it eventually clears up. I'm kinda confused. Is this an exhaust issue or something else? Please help!