Possibly a carbon build up problem as suggested above. I recommend using fuel system cleaners, a can of carb cleaner, a full tank of Cheveron Supreme, and SeaFoam. Water works well too, but if your pistons looked anything like mine, then good luck. My truck was dieseling before I changed the head gasket, if I layed into the throttle after I cut the ignition. When the head was off there was a fat layer of carbon on all the pistons, about a 1mm thick, maybe even thicker. I cleaned them up and it dosen't diesel anymore.
