This is a fluid pressure problem within the transmission or in the torque converter (which is external to the transmission but connected to it). It could be simply dirty fluid or a clogged filter which restricts fluid flow within the transmission and a fluid and filter change will fix this. If there is a dirt clog in the valve body, you probably need a replacement valve body since it's very hard to find someone who will rebuild a valve body on a ZF. If it's the torque converter, the torque converter should be replaced before the slamming damages the transmission clutches. The automatic transmission depends on free flow of the transmission fluid through the valve body so that the proper gear can be selected at the proper time, all of which is controlled by fluid pressure. When there is a small blockage, the transmission can't change gears in alignment with engine speed and you get the slamming. Slamming is a different problem than slipping which would indicate that the friction surfaces on the clutch packs are worn and heading to failure.
It seems that new car dealers, particularly the European brands, view transmissions much light bulbs: If it doesn't work right, screw in a new one. They don't seem to have much interest in what happens inside the transmission case or any idea how to fix, or even diagnose, an internal transmission problem. There must be some economic or political reason for this lack of knowledge or interest. I don't know what it is. The best bet is to find a really good transmission specialty shop that has experience with ZF products. A fluid and filter change is probably only a couple of hundred dollars and it's important that it's done correctly since some ZF transmissions need to be filled according to the temperature of the fluid which is monitored as the fluid is added.