Leaking transmission

I have a 1993 Taurus 3.8 V-6, and twice now within a month I too have mysteralily lost all (5 qts) of trans fluid. Trail down the street, huge puddle in driveway, and both times after thruway driving and doesn't seem to dump out until I come to a stop.. Not an issue at city speed, or short trips. I had bi-passed the entire transmission cooler asembly because of a leak in it before this starting happening. After my mechanic too was at a loss, he thought it might be the fluid overheating, causing excessive pressure and the fluid coming out of a transmission vent? Suggested I reconnect the transmission oil cooler assembly, and drive for a while, otherwise I may want to try a flush because the filter could be plugged he said. The repair was just done today, so I can't say for sure thats what it is, but I will follow up. I have been fixing cars, engines etc.. for 25 years and have never encountered this, then again this is my first Ford that I have owned. Good luck.