had a problem like this before, i was sucking in air while driving and had to pull over and re-prime the system got to a point that i was doing this about every 2-3 miles, i cut all the metal lines and replaced with rubber lines and i pulled the sending unit out of the tank and lined the tubes with rubber lines, it fixed the problem, there is a pin hole somewhere in the fuel system where it is in the tank or on the line themselfs, id check o rings, my newest 6.5l had the same issue but there was no o rings on the line going from the tank to lifter pump. hope this helps