how can i fix my mercedes vito?

hi, i have air and foaming in clear fuel line going to inj rail on w reg mercedes vito diesel. i have replaced all pipes and seals to high pressure pump and low pressure pump. i have replaced fuel filter with an o/e one. added primer pump to prime diesel to rail. it will then start ok after 5 /6 seconds, run for poss 5 minutes ,then air bubbles start to appear from middle pipe of fuel filter which goes to rail. it obviusly then cuts out. please advise where to look next, its not the high pressure pump as i have changed that off another vehicle that was running fine and to rule out the pipes to fuel tank i have run them to a gallon can of diesel and problem still there. thanks