TGM 18.280 Problems with unround motor

Hello, since January I have a TGM 18.280 and in the lower rotation range the engine runs round and does not sound good at all. After the replacement of the ijectors, however, nothing has changed and the engine data are not noticeable. The workshop has tried to fix this shaking already three times, without success. Does any of you have similar experiences with the engine?? The error occurred already at the handover of the vehicle and is up to now unchanged. Greeting Horst35153