I recently aquired a 380 SL (74,000 mi) with the harsh shifting problem you describe. I was advised to do a transmission service b/4 anything else. Consists of changing fluid AND filter. That made an immediate improvement and the shifts seem to have gotten smoother since then. It's still not what I would call silky smooth, but I don't think that's the nature of these transmissions.

There is also the transmission modulator adjustment that you might look into also. (adjusts the pressure)