Peugeot 206 1.4 hdi keeps cutting out, but starts up again ok then cuts out?

I can see a few answers on this blog about the peugeot 206 cutting out for no reason, mine is doing the same, crank sensor replaced, no joy, new filter no joy, thought it might be the key so used master key, no joy, any ideas, it is dangerous as people say its like someone just switching the car off no matter where you are. Ideas please.