This sounds like your alternator is having some problems.  Mine did the same
thing when it died.  You can take the voltage regulator off and look at the
brushes inside to see if they are OK.  But I would recommend buying a used but
not abused alternator from me.  I can get one for you for only $50.  This is
cheaper than you will even be able to buy a new voltage regulator.  I can ship
it to you anywhere in the US, COD.  If you are interested please send me your
zip code and I'll get you a price with the shipping.
Don't take my word for it though, take the alternator to a shop that rebuilds
them and have them bench test it.  They can tell you if it is bad or not and
how much it will be to rebuild (more than $50).  I would take the van off the
road because very soon you will try to start it and it will not start due to
battery drain.  Mine ran for about a week before it finally was totally dead.
Ken Wilford
John 3:16