Marcus,

It sounds like the classic vanagon wiring harness problem.  There are
several approaches for the fix in the archives.  The definitive test is to
stop when the problem occurs, and turn off the vanagon.  Then just restart
it and continue on your way.  If the problem goes away for the next hour,
you have the infamous problem, otherwise?

Mike

Michael A. Radtke  -  Z74               Voice: 602-862-4897
Bull                                      FAX:         4853
13430 N. Black Canyon Hwy.
Phoenix, AZ  85029                      Email: m.radtke@bull.com