Wet weather haltering under load

[Joel Skousen <jskousen@enol.com>]

My son came into the Audi quattro club this month with a used 87 5ktq.
While test driving it about two weeks ago, we noticed some haltering
above 1 bar under full acceleration.  It isn't real bad, just a
noticable cutting out for very brief intervals, only under heavy
acceleration. The owner, who is a personal friend, said he had not
experienced this--then again, he doesn't run around testing his car at
full throttle either.
  After buying the car, my son could not get it to reappear except when
some kind of moisture was coming down from the sky.  The day we test
drove the car, it was raining too. He just drove it out to California,
and everything went well for 8 hours until he went through a storm over
the Sierras, and the haltering under heavy load showed up again.  Any
ideas of what kind of thing may be humidity or moisture related while
driving under heavy load?  Anybody else been there? Thanks in advance, 
Joel