I've seen this fault mentioned all over the net - on '99 DI Transits in particular (symptoms of air trapped in fuel lines).
Almost at my wits end, I tried a mod suggested elsewhere on this site, moving the leak off return hose to the front injector (from the back).
It seems to have cured it (been about a week now with no problems - before it would happen on almost every trip...)
Not before, i might add, I had spent heavily on injector pump "repairs", new fuel filter (probably a good idea anyway), and seriously considering selling it to some other hapless soul...
You live and learn.
Many thanks to Rob for the mod.
Owen
October 2006