Have you identified which cylinder is missfiring? Slack off the pipe between the pump and injector and this should confirm which cylinder is doing all the work. If could be an air leak on the diesel inflow pipe. Also do you have a gravity fuel feed from a day tank or do you have a fuel pump supplied by the main tank. these have been knwn to cause problems? I have had one where overheating caused a misfire caused by a broken water impellor. The injector cleaning is a sound idea, You can compare spray pattens if you crank the engine over whith the injectors removed but be careful not to have your hands, bare skin or eyes near the spray, diesel is nasty stuff.