Stumped - Transmission and a rainstorm

[Clunkers]

Howdy - car details: 2002 SC2 162k miles on it, automatic transmissions. I'm very particular about taking care of this car, as they are harder and harder to come by.

I replaced the valve body with a rebuilt about 7 months ago, as I was having some gear slamming and holding onto gears. No issues for quite some time after that. Recently though, we had 2 good rainstorms in the past 2 weeks. The first rainstorm I drove through, I noticed the transmission acting up; slamming shifts, holding onto gears and just overall acting up. I thought, well maybe it was a fluke, but kept an eye on it.

The second good rainstorm last week, SAME exact thing happened while I was driving through it. It was like a completely different (horrible) transmission.

No dash/idiots lights on. So now I'm thinking electrical gremlin. But I'm not even sure where to begin with this. I do live in a snow region, but I got the car a few years ago from a southern state that doesn't have snow/road salt, so its almost 100% rust/corrosion free.

Any ideas or thoughts would be greatly appreciated. I've had suggested to me possibly the ECU or alternator.
Thank you for taking the time to read this.

[1993sl2]

The manual transmission fill plug on my 1993 sl2 dried out and didn't seal well. When it rained really hard I got water in the transmission. I had to change the fluid for it to (mostly) get better. Sounds like yours gets better when the rain stops.