My P38 has developed a problem which makes the car stop as if starved of fuel every few miles. Having just had a new diesel fuel filter fitted it has now developed this problem. The car has a hand pump / bulb in the fuel line just before the filter and when the car stops has goes hard as if pressurised. after a while it will go down and I am able to hand pump fuel again, which allows the car to start, but only for a short time. my local specialist has been unable to diagnose the problem, please help