Vertuas,
Over the last few days I developed _exactly_ the same symptoms as you described with my TD4.
It was diagnosed at the main dealer as the under bonnet fuel pump (located beside the fuel filter) not always coming on. You could tap the body of the same to fire it up so the car would start. Once that was done the car would start and keep running ok. Since the unit has been replaced the car has started and run fine.
If you are still trying to fix this problem this information should help.
Richard