1999 Ford Taurus transmission fluid leak

I run book routes in my 99 taurus, approx. 500 pounds of books, constant stop and go traffic all day, after several hours trans fluid seems to leak from side or bottom of trans. I was told the car was not designed for this type of operation and the trans fluid is overheating, I was told a solution was addng an extra trans cooler. Does this seem like sound advice to you? When I use the car for normal everyday use there are no fluid leaking problems! Thanks for your help!