Check out the technical bulletins - see if any apply to you. I see a couple candidates..

http://gerry.vanagon.com/files/technical/VW-Technical-Bulletins.txt

My '87 sofabed had what they call in the bulletin 'hestiation or surging' on the highway for years -
nobody could figure it out.
everyone I asked suggested the AFM, even fuel filter, etc.
Finally someone suggested the service belletins - there it was -
 the wire harness Part No. 025 906 302 fixed it and we haven't experienced it again since.

So now all I have to worry about is the coolant temp. - it steadily increases in traffic, despite the electric fans functioning.
While on the move the temp is stable, in the middle of the gauge. So it seems maybe the readiator is only doing its job under the most favorable circumstances? ... maybe I need a new Copper Core German Radiator?