Engine Performance problem
1994 Ford Explorer 6 cyl Four Wheel Drive Automatic 125000 miles

My 1994 explorer engine light is on.. I drove it like this for some time. The other day while entering the hwy from the ramp the truck would olny go 35 mph.. Prior to this when on the hwy at 65 mph the engine light would go on and off periodically.. When the light would come on you could hear a slight thump (like it was shifting gear).. when turned over the exhast lets out a puff of black smoke.. If placed is park while running and the gas pedal is pressed black smoke come out of the exhast..