One, fuel filter in the tank is plugged. Two: electric fuel pump is going bad. Three: fuel filter on top of motor is clogged. Four: HPfuel pump is going south. I had the same problem with my 99 7.3 at 265K miles, it was both the first two things listed causing the problem. It will run bad when the oil gets dirty and the oil filter starts to get plugged too, this reduces the oil pressure to the HP fuel pump causing a fuel pressure loss at the injectors.