1994 S500 Coupe, Downshifting

[BenzPrincess]

I have a 1994 S500 coupe. It started downshifting on the freeway. I will be driving on the freeway, it shifts to 4th gear and then when I slow down a little, it will downshift to 3rd gear and will not upshift. I have to stop, turn off the car, and when I turn it back on, it will go up to 4th gear. God forbit I slow down again, it will downshift again. I checked the transmission oli and it is full. I am not sure what to do. I asked my mechanic and he said there is nothing I can do. It sounded like he was trying to get rid of me. Any advice?