TDI 2.5 d discovery?

Starts and drives but when you drive for about 1/2 mile or so and pick up speed she just cuts out ! I have put a new fuel filter on and fuel is flowing ok so why does she keep doing this? If I try and start her again straight away after she cuts of she only seems to be fireing on 2 cyilinders but if I wait for about 5mins she starts and drives but only covers about the same distance. I'm so stuck and just don't no what it could be can anyone help.