I got a problem with my 89 190E gas/auto transmission.

If I drive on the freeway for about 20 minutes. Trans fluid starts leaking from the trans. It doesnt leak while sitting so that rules out pan gasket, what else would make it leak while driving in 2nd gear = 50-60 mph?

Any ideas, It just had a fresh fluid and filter change, I found nothing out of ordinary in pan. I am trying to sell the car, but this would be a problem for a new owner.

The car has 194000 on it but I dont know how many miles are on the trans specifically. The fluid basically is leaking off of the pan edges at bottom when I stop and look at it. It is also smoking a bit from under the hood so some of the oil is being blown to the exhaust pipe.
Any thoughts.

Thanks.