Suggest you look for an air leak into the fuel system - perhaps just
before the injection pump.
If air is drawn into the fuel system when the engine is shut down, then,
upon start up, that air will enter the injection pump and prevent the
pump from delivering full opening pressure to the injectors. After the
tiny air bubble has been expelled the engine runs normally.
Check the fuel line fittings, filters and clamps etc. in the supply line
before the fuel pump and injection pump.