Transmission problems ONLY when wet!

Hello, long time reader, first time post.
I was hoping someone might be able to offer advice on my problem.
I have a 1995 Tacoma, auto, 2.7L with 215K on the clock. I maintain it well and regularly change all fluids and other maintenance items when scheduled. I have this problem which has just started.
When driving in rain the trasmission doesn't work properly.
- It will stay in 3rd when I slow down, stop and then start again.
- It will intermittantly change down gears while driving on hwy.
- Feel real "Chunky" / "Clunky" like it is trying to shift into/out of another gear.
- Although, will shift no problem when I manually shift through the auto transmission
After I let it sit and dry out for a few hours it is fine. Drives beautifully like nothing is wrong.
I have checked what connections I can find and applied dialectric grease thinking that I am getting a short somewhere and no luck.
Please can someone offer advice, we have had plenty of rain and more comming!!

 

Sounds like a connection somewhere is getting wet. Possibly naked wires? Crawl under and check the connections is what I would do.

 

I did. I checked and applied dialectric grease to what wire connections I could see. Maybe there are more which I can't see. There is very little room on top between trans and under body. I'll have to find a way to look further.
Thanks anyway!