when you installed the fuel pumps did you use the original rubber line connecting the sending unit and the pump? i had a customers car that acted the same way, turned out to be a small leak in that rubber hose. when the tank was full, the line was submerged and couldnt draw in air. as soon as you drove 30 miles or so and used a little fuel, the hole was exposed to the air pocket at the top of the tank. just an idea. g/l