William,
   Interesting theory.  Watching my O2 sensor  monitor gadget
http://neksiwel.20m.com/photo5.html , I notice that rhythmic idle
fluctuations correspond with changes in the output of the O2 sensor. On my
truck, they are more noticeable while cruising slowly in first gear at idle
speed and barely detectable just sitting at the light.
   You did not say how badly it was hunting. On our 85 I could cure this
hunting my turning the idle bypass screw inward (CW).
   While we are throwing theories around, how about a bad chassis to engine
ground strap? The extra current of the fan could be dropping a voltage
across the poor connection which might add or subtract from the O2 sensors
output.
  To test this theory  I would turn on as may electrical consumers as
possible and measure the voltage difference between the case of the
alternator and chassis ground. Should be as close to zero as possible. My
truck was loosing almost a volt here and I actually added a hefty wire
between these two points.
   While you're at it, I would also measure the voltage drop between the
positive battery terminal, if possible, and the output of the alternator.
Some older Vanagons had an underrated wire between these two points.
Ken Lewis
http://neksiwel.20m.com/