I had the same problem, and as odd as this may seem, it was a fuel problem!
There are two plastic tanks that sit just behind the front wheels.  These
tanks are connected directly to the fuel tank (I am not sure of their exact
purpose).  There are seals on the top of the tanks.  The seals failed, and
spray from the tires went directly to the fuel tank.  The fuel filter would
become waterlogged and plugged.  This not only resulted in poor performance
during the rain, but fuel line freeze up during the winter.  You should be
able to tell if this is the problem by checking the fuel filter during
problems.  Good luck.
Vanagon40

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