My truck, will start, but it only runs for a mile or less and it starts slowing down, it does not want to go, sometime it barely moves, my husband change out the fuel pump, the coverter or what ever, then he change the plugs, he says when he changed the plugs it ran a little further but then it did it again and barely made it home, the truck is a 2006 C 60 series and has a 8.1 engine, gasoline, and vin # 1GBJ6C1G96F410936 and has 138962 miles, help,