I have a 95 mercury sable 3.8 liter and when I drive for too long in warm weather, it leaks tranny fluid

This is an interesting one. Where is the transmission fluid leaking from? If it is coming out of the transmission where it meets the engine, then you are looking at replacing the front transmission seal, and to do this you will need to pull the transmission. If it is not leaking at this spot, then you are looking at a line leak, and will need to replace the bad line. There are going to be two lines for the transmission, make sure that yoyu inspect each one of them thoroughly/ They are the usual culprit when a transmission starts to leak.

This sounds like the trany is overheating, you will need to have the cooler and lines checked, make sure the trany lines going to the radiator are not kinked and are not bypassed from the radiator, a good idea may be to install an aftermarket cooler for the trany, this will bypass the cooler in the radiator, this is a small radiator type cooler and the trany line are connected to the cooler and cooler is then mounted in front of the car in front of the ac condensor and will help keep trany oil from overheating, good day.