Technical Idling problem

[apothecary]

I've got a 2004 Ducato 2.3td. It won't idle. After a couple of seconds, it cuts out. I then have to wait about 5 seconds before I can restart it. If I keep the revs up, it's fine. And to be awkward, I can coast in neutral and the engine idles perfectly! However, if I'm at a stop, within a few seconds, the engine cuts out. Anyone got any ideas as to what's wrong?