Does this happen when the van is cold or when it is hot?  If it happens only
when the van is hot then it could be a case of corrosion in the wires or bad
connections.  If it happens any old time then either your switch is going bad
(which I doubt since you hear the fuel pump come on) or your solenoid is
sticking.  I had this problem on my '84 and solved by removing the starter and
then opening up the solenoid and cleaning it up with some scotch brite and
WD-40 and then putting it back together.  Has worked every time since then (5
months ago).
Hope this helps.
Ken Wilford
John 3:16