I had a similar thing happen to me with my '84 Vanagon about a year ago.  I
posted it to the list at that time but here is what I discovered:

One of the lines that go from the gas tank to the expansion tanks in the
front wheel wells was old and had started to leak.  I replaced this line and
after that the van ran great.  I tried telling folks about this last year and
asking what could've caused the van to run so terrible just because one of
the fuel lines to the gas tank expansion system was leaking and no one ever
really came up with any ideas.

Hope this helps,
Ken Wilford
http://www.vanagain.com
John 3:16
Phone: (856)-765-1583
Shop:   (856)-327-0027
Fax:     (856)-327-2242