SO if I understood well , when full , you have lack of power and when low your van is normal?
Might be worth checking , after a long run and full tank , go and open the fuel cap and listen carfully if as you open it up it sucks air in Make sure the breather pipe of tank and cap has a small hole for air passage, so the engine will not have any problems sucking the diesel.
Else check the return pipe if its blocked
Keep us informed how you get on.
And WELCOME as I can see this is you first post