Sounds like your ECM is getting water in it.  I had a squareaback wagen that did that once.  Be careful, or you will fry it

Chaz
'82 Westy "Peewee"
Finally running

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From:   John Rogers
Sent:   Friday, August 07, 1998 8:33 PM
To:     Vanagon@VANAGON.COM
Subject:        Afraid of the Water

May the brainpower of the Vanagon Collective advise me....

My 84 Van can't tolerate wet at all..it stumbles and bumbles at the
first drop of rain,even after I wash it it will run on 2 or 3 cylinders.
So I got new plugs and wires thinking the easy fix would do it, of
course it didn't fix it. Runs fine when dry, temp gauge always normal,
I've never even heard the fan come on..any hints???

Thanks as usual!

John R.

84 GL 92k
80 KZ1300