Once I had my coolant changed on my S420 at dealer. My car used to hit 100 but after a couple of months, the temp has now settled to 80C. My best reasoning is 1. air bubbles 2. coolant not homogeniously mixed completely 3. too much coolant. 4. Additives in coolant.
Sometimes, engine overheating has nothing to do w/ the cooling system but the oil pump. Oil is also recirculating throughout your engine causing cooling. If the pump is bad, the engine will heat up.