If it was the pipe arraangement problem you would expect the front (nearest the front of the van) cyclinder to misfire. As the engine slopes backwards slightly this is the one which is highest. Normally the air acumulates when the engine is stopped so your engine is fine one day and bad the next.
Have a read here:
http://fordtransit.org/forum/viewtopic.php?t=747&highlight=injector+return+pipe
Steve


