check your fuel cap is a vented one and the vent is working,or if someone has put on an unvented cap the fuel tank will create a vacuum which , after a few minutes equals the suction of the fuel pump.when this happens the car runs out of fuel till it sits till and the vacuum disappears ,at which time the car will start and run for another few minutes.something may also be floating about in your tank (rag,plastic bag) or floating about in your float chamber (small piece of fuel pipe rubber which can corode internally on volkswagens).when i worked for vw we had a litre plastic container which fixed to the nearside front door window attached to the inlet side of the fuel pump ,this not only eliminated any fault with the pipes or tank (as yours may be) but we could easily check mpg.hope this helps
philky
September 2008