Is it the entire seat or just the seat back?
I had a similar problem with my 1987 300E. My problem was the cable for the seat back jumped the track. When I would operate the seat back switch the seat would lean to the center of the car just as you describe. At first the MB tech wanted to replace the cable so he ordered it. Lucky for me the wrong one came in. He lubed my cable reinstalled it and it has been working fine and that was two years ago!!!