Check the ignition coils...especially at the spark plug wire connections.
I know it's a stretch, but my old VW had the same problems whenever the environment was moist...I finally traced the problem to the ignition coil...corrosion was discovered at the main plug lead.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car