I know exactly what it is.  The diesel fuel, when it sits gets gummy.  I have
had this problem several times as we have a large VW diesel fleet on the road.
How I fix it is get some Fireball diesel injector cleaner or something
similar.  If you can't find diesel injector cleaner then just use ATF
(automatic transmission fluid) it is basically the same thing.  Get a oil
bottle.  Put a mixture of the injector cleaner and diesel fuel in the bottle.
Now get the plastic top that comes on transmission oil contaners and has a
sharp point on it.  Hook a short line from the bottle to the supply side of
your diesel fuel pump.  Now as you are attempting to start the van squeeze the
bottle.  It will still start hard and you have to let the van run for a while
with the injector cleaner in the fuel, but eventually it will start up and get
back to normal.
It could also be that your solenoid on the fuel pump that allows fuel to flow
through the pump could be gummed shut.  It is right on top of the pump and has
a really thin black wire running to it.  Unscrew it and try to clean it off
with some clean diesel fuel.
Of course there is the possibilty that your problem is more severe but I have
had this same problem so many times that I am also positive that this is your
problem.
Let me know how you make out.
Ken Wilford
John 3:16
'81 Jetta diesel, '85 Jetta diesel, '86 Jetta T. Diesel, '81 Nissan Maxima
Diesel (great car and a 6 cylinder!)
And of course Vanagons as far as the eye can see!