Sorry to disagree, but the fix is to replace the Valeo radiator, which is a well known problem on the '04 203/209 MB's. This will prevent the contamination issue from re-occurring. Flushing the transmission and cooler lines will take care of the symptoms described by the OP, which is caused by contamination of the torque converter lock-up clutch. It may require a second or even third fluid change, but the transmission will be OK.

Of course this assumes that coolant contamination is what's causing the problem in the first place. A faulty transmission ECU can cause the same symptoms. I've seen both scenarios play out many times............