Just a note:  I had a Volvo sta. wagon years ago.  When high humidity (often,
on the Gulf Coast) was present it would not start.  Everything was trying =
starter, distributor, Battery, and carbs.  The dealer placed caps used for
marine applications over the sparkplugs = no more problem.  The spark was
grounded to the block.  May not be the same problem, but consider the possible
places that the spark may be grounding and clean/dry them.
Don
84 Vangeaux