my 98 2.5 diesel lwb transit occasionally lets fuel run back to the tank?

My 98 2.5 diesel transit seems to occasionally let fuel run from the pump back to the tank, so when you try to start it it's as if you have run out of fuel, it does eventually start but only runs on 2 or 3 cylinders until it's bled it's self which can take anything from 5 to 60 seconds. After this it runs fine, it happens on a random basis, could be twice a day or once a week. I use it daily doing 1000 miles/week and it usually starts first turn