Steve, check out my article about "bucking syndrome" on this web page:
http://199.217.188.30/disc2_frm.htm
I have helped many a Vanagon owner troubleshoot his own system with a
multimeter and a Bentley Manual.  Check all the values with the meter and you
should quickly find the culprit.  I have all the parts you need to fix the
van available as well so give me a call when you find out what you need.

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