Engine cutting out

I've a discovery 300 tdi. its taking 3 or 4 turns of key to get engine started. when engine is running, if engine is revved up then allowed to drop back to tick over the engine just cuts out and then takes another 4 or 5 turns of key to get engine started again. I've changed fuel filter, but no change. Anyone any ideas of what prob may be.

sounds like air somewhere to me(in fuel line)-or solenoid playing up

or sediment filter blocked.