Hello,Such problem occurs due to aged o-rings between the fuel filter and high-pressure pump. They let the air in and allow fuel to flow back. Please have them all replaced.
Next possible cause is one of the injectors leaking too much fuel into the return. In this case injectors are not activated because starting pressure of 200 bars is not reached . So next step after o-rings would be to test the injectors at bosch diesel center. They can rebuild them if some are faulty.