HI We recently had a vivaro van(same van different name) with same problem as yours. I grafted a peugeot hand prime pump onto fuel pipe(replaces original pump) this was done as a temp measure (saturday morning job) this cured the problem never seen vehicle again. new pipe and pump from vauxhall was about £20-£30 (still sat waiting to be fit) I think the moulded plastic pump gets a weak spot and gradually sucks itself together stopping fuel flow.