The modulator valve on the transmission controls the firmness of the shift. The modulator is vacum controled. The vacum controls on the diesel engines are located on the valve cover and on the injection pump. The less vacum the modulator valve has going to it the firmer the shifts are, so it sounds like the car might have a vacum leak in that system.
I would have it looked into pending the purchase of the car, usually it is not an expensive repair to correct a vacum leak in that system. What you don't want to find is that someone adjusted the modulator valve to cover up a slipping/dragging shift problem due to wear in the transmission.
Hope this helps,
14 year MB tech
7 year shop foreman @ independent MB/Volvo shop