Citroen relay van after driving about 15 -20 miles cuts out even on motor way leave for couple minites and will start. Then cuts out every 5-6 miles the whole journey
Sounds to me as if a fuel tank breather pipe is blocked and is creating a vacuum in the tank as you drive along. When you stop try taking off the filler cap immediately - if you hear a hiss of air that should confirm it.
Clear the breather pipe of any obstruction or kinks to give a free flow of air.