No idea .. there are a lot of faults with transits that do just what you are saying, the solution is often long and drawn out and sometimes there isn't one and the thread just goes away.
They will probably start off by telling you to check the fuel filter isnt leaking air into it
Then there is or is not, depending on year, an in tank fuel pump, these begin to jam intermittently after a few years. when they do bash the tank with a big rubber mallet as a temporary fix.
Or according to poster here FRAG the other day ... if it has an in tank pump .
"Likely the fuse terminals that feed the fuel pump relay,need closing up.
Common fault,"
all the best.mark