How can i fix my fuel problem on my 1993 Renault espace 2.1rt diesel?

Hi I have a 1993 Renault espace 2.1 rt diesel it runs for a short time with no air entering the fuel pump from the fuel inlet pipe however it suddenly starts to run very rough and finaly conks out. At this point a large amount of air rushes back from the pump through the fuel inlet pipe and back towards the filter.
I am wondering what could be failing on the fuel pump to allow the air to leaking into the fuel system.
Grateful for any ideas folks
cheers

Dave Brown
April 2009