Thanks for the post! I'd' definitely consider the notion that a more consistent climate may not experience the season gremlins to the same extent. Pretty dramatic difference in the environment inside the house between summer and winter in the midwest. I've thought anything from airborne pollen and such, to differences in groundwater may be some part of the reason, but I may never nail down exactly what happens during seasonal swings.
Do you live near fields or something else? I have friends of mine having issues with reptiles on certain period of the year due to some products on fields treatment for example. Also some RO water could be contaminated. For example some friends of mine in Paris, after the terrorist attacks in Nice has experienced strange things in their tanks due to RO water. To prevent poison in the water, they put products to kill all potential poison but it can be rought. Like you sometimes have a letter saying "don't give kids or women wating for baby this water". Even if you check your conductivity some elements are still not filtered by your RO/DI unit; and it can be polluants.