Transmission probliems

I had my transmission rebuilt, my radiator replaced about 2 years ago. When I picked it up from the transmission place, the car ran great, about 2 months later I returned there because I was getting a P0700 code. After about 2 months of returning to the mechanic it was finally found to be either the input, or output speed control sensor, I replaced both of them. The beginning of this summer while sitting in traffic everything seemed fine until the car needed to shift into 3rd gear, it would not. All it did was race the engine high and finally slammed into gear. I got home, checked the fluid in both the transmission and the radiator, both were topped off. Now once the engine gets warm this seems to happen, believe it or not, only on hot days. I can go 2 weeks never see it, then the sun is out all day and BAM the problem shows back up. What could this be?