Why does our truck run rough/sputter in the same place on the same highway and run like a top elsewh

Have a friend who had an 80’s something Buick wagon,that had some kind of a warning light that would come on,when He passed an extrusion plant on the way to work(so a person cant rule out enviromental factors) anyrate on the aforemetioned Buick it had one of those troublesome electronic carburaters and my older brother rebuilt a Q-jet and installed it for the neighbor and the car ran perfect from that day on-Kevin