'85 500SEL Transmission

Hello,
I bought this car in August of 1997, and it had 95000 miles on it. It ran very well for about a year. I then had the transmission serviced by a local mechanic since I live way out in the country. The next day, after the driving the car for about 100 miles, I parked it, went shopping, and upon my return noticed a huge trasmission fluid spill underneath the car. I had it AAA'd back home, and the mechanic reserviced the transmission, this time tightening everything properly.
About a month or so later, I noticed when the car had been running about 100 miles or so, the engine started racing and the RPM needle showed the revolutions to be at 25-30000 in the second gear. I can make it drop down by shifting into neutral, slowing the car down, and then putting it back into drive. I know it can't be good for the transmission to do that, but I don't know how else to deal with it.
The local mechanic told me it probably needed a new transmission, but someone else said that it might just need to be serviced once more, replacing filter and gaskets. Could it be anything else? Please help. Thanks.