308d fuel problems i think ??

Hello can nyone help me.. I recently bought 1999 2.3 308d with the hope of converting it, when i collected it it had 3/4 tank of fuel everything running well . a couple of weeks latter and a few trips it was on under a 1/4 it seemed to run fine for 1/2 a mile then stop, it would take a lot of cranking over but then start and the same again stop.. got it to a garage who thought it might have been a crankshaft sensor but the diognistic machine shows no faults. i have noticed that when its running and i rev the engine i can see air in the fuel line which go between the filter and the pump . also when it stops i've checked the fuel filter and the fuel level in the filter is only80% to the top . . ive changed the fuel and air filter .. any ideas ??
cheers Neil. also if anyone knows a good mechanic in cornwall, as i've had no luck finding anyone who will even look at it..

 

make sure the fuel pipes from the tank to the engine are on the right way firstly. if the tanks been off this can happen.