Had a similar problem with mine after the trans fluid/filter were replaced. Shifted fine before, but since I just got the car I thought I'd get 'er up to speed by changing the fluids. Had to take it back twice and have fluid suctioned out. After the second time, she started shifting smooth as silk.
Could be any of the problems suggested by the previous posters, but it could be something this simple too.
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000