Fine..That removes one possibility, most likely. And, I believe you said
that you had new spark plug cables
as well...

I still suspect something in the ignition, but dont recall any sort of
sensor that would be likely to be so
humidity sensitive.

If you have the HEI ignition, I would pull the coil and check it carefully
for arcing.  I have seen this happen
a number of times, and it can kill the ignition, some of or all of the time.

Can you replicate the situation with a mist bottle when there is no rain?
If so, can you check your spark
to plugs, maybe using a cheap spark gap plug tester, before and after
misting around the engine compartment.

I know how frustrating this can be.  And sometimes the trees obscure the
forest.