125,000 miles, autotrans. Today, while towing 5th wheel, at 2,000 rpm, 65mph, when encountering slight grade, would mis-fire, black smoke from exhaust. Laying off the accelerator will make the problem go away, as well as increasing accelerator, forcing downshift and rpms running upwards of 2,500-3,000, problem also goes away. Will happen at slower speeds, but generally at higher speeds when under strain just before downshifting. We had driven all day with no problems. We had fueled up about 50 miles before this happened. Bad fuel? Injector problems?