1993 Ranger transmission over heats and boils out when ambient temp is above 95.?

Living in Dallas, we will have at least 100 days of temperature above 95 degrees. My job requires the vehicle to idle for 3 to 4 hours per day. I replaced fan clutch so the engine runs cool, but once the water temp get to about half way, and it is above 95 degrees, tranny fluid starts to leak out. I can drive down the street, and vehicle cools right down.

It has 100k miles and I feel certain the filter and fluid has never been changed. Well, I have replaced at least 15 quarts that have leaked out, but not the filter.When I drive back to the office and park, no fluid leaks on the ground.

Should I try and find a larger transmission cooler, or will a filter replacement do the trick.

I will open a cold one and await the proper answer.