I was driving my 04 GMC 2500 HD Duramax/Alison for the first time today on the freeway and had an un GM like experience. The truck is an all stock 6600 LLY with 250 miles on it and after driving for about 45 minutes at 65 mph, I notice this thick black smoke coming out of the exhaust pipe. If I keep the accelerator steady, not as noticeable, but still smokes, however, with the slightest excelleration, massive black smoke. I thought maybe there was some carbon buildup so I put the pedal to the floor to blow it out and again massive black smoke and I hear three or four clicking noise in the engine compartment along with a "bucking" sensation. I pull over to the side of the road, check under the hood, restart it, get back on the freeway and the truck runs almost perfect. Very little smoke, and only if I stomp on it. Runs great another 45 miles and the exact sequence happens again. Smoke, runs rough and then after shutting it off and restarting it, it runs fine. I dropped it off at the dealer Sunday night. I'll post back what they have to say. Probably fuel injectors, I think. Anyone have any other ideas?