I had a problem with my 95 Safari, Cummins 300. It would run fine for 500-900 miles, or sometimes only 2-3 miles than when I would come to a stop,it would loose all power and black smoke would pour out of the exaust . I was towed in twice and nothing could be found. We changed filters and it would run fine for a while. Well to make a LOOOONG story shorter. This was over a 15-18 month period. It finaly acted up and stayed broke. The villian was a sticking electric/neumatic valve controling the exaust brake. When it would act up, and when I stopped and changed filters or was towed in it fixed itself by leaking by. Thank goodness it finaly stuck and stayed closed. Hopefully this will help someone down the road.