Transmission leak

I have a 99 Legacy L that I bought used at 44,000 miles. It has 73000 miles at this point but I have twice noted that when I drive long distances on the freeway it the transmission tends to leak and smoke rises when I stop. I initialy thought it was the breaks but on a carefull look I saw the smoke comming from under the transmission. Please help with any advice. I am contemplating going to the dealer but I would like to have a clue on the matter first. This only happens on long hauls of like 100 miles or when caught in trafic jams. The transmission is also getting increasingly pingy on shifts and has hard shifts. The gas mileage is low on this car.
please help