1979 Cadillac Seville Can it be the map sensor?

It all started when I drove the cadillac back from San Diego to Phoenis. Approximately 3 hours into the trip, the car all of a sudden would not hold the speed, what I mean is that the car would gradually sloww down, unless I floored the car up to the highway speed, but then, when I would let off the accellerator, the car would lower it's speed again. Then, once I stopped the car and shut it off, it would start right back up and runn fine in idle, but, as soon as I put the car into gear and tried to put pressure on the accellerator, the car would act like it it is choking itself out. I have a 1979 Cadillac seville that runs extremely rich and after about 20 minutes of running in idle, the cadilettic convertor gets hot and smoke starts to surround the car.
I would like to know if it is the computer under the dash that is causing this or if it is a minor adjustment to the throttle body.
I have replaced all the vacuum lines.