Transmission slipping in and out of third

My transmission (automatic) started slipping in and out at about 35 mph. Once I hit about 45-50, it was fine. Had transmission rebuilt, and it kept doing the same after about two days. Replaced a barrel in transmission, after 24 hours, back to slipping. Completely replaced transmission with another rebuilt, drove about 360 miles (on a road trip... mostly interstate driving) and it started again. Not nearly as bad. Coming back home, in the middle of a nasty rainstorm that slowed interstate traffic to a crawl, it started slipping worse than ever. When the storm was over, and the truck seemed to dry out, it stopped for the most part. Back in town, it started slipping at around the same speeds (in and out). Now, when I come to a complete stop, it doesn't want to engage and I have to pull over and tap the gas until it engages and I can drive. Then it starts slipping in and out at the same speeds.
Seeing as how this is a completely different transmission, I feel that the issue isn't in the transmission itself, but something causing the transmission to start slipping gears. Also, It seems to be better when it is warm and dry. If it is moist outside, or cold, or hot, it starts slipping. Cooling temperatures, morning dew on the ground, and a couple of days of rain happened before it stopped engaging after a complete stop. This is why I believe that there is a moisture issue somewhere.
I really cannot afford a new transmission again, nor can I really afford to take it to a mechanic this time, so I'm hoping someone can give me some answers that won't be terribly expensive.

Just FYI, the transmission fluid has been fine every time it has been checked (This has been checked by my mechanic, myself, and the transmission mechanic several times.)