Cylinder missfire
To detect a missfire, you could slowly loosen each injector line to see if the way the engine is running changes, but an easier method is to disconnect the glow plugs and measure their resistance with the engine running (the glow plugs have to be in good condition). As the engine gets hot, glow plug resistance increases. A cylinder that isn't firing will be colder than one that is. After starting, glow plug resistance usually rises to 15-20 ohms on a operating cylinder, but stays below 10 ohms on a weak one--even lower if the cylinder is dead. If you own an infrared temperature gun, you can detect the missfiring cylinder by measuring exhaust port temp. If the miss cannot be isolated to any cylinder(s), then there may be an injection pump or fuel quality problem.