My first guess is the radiator. I have seen many of them plug up in the middle...you can't clean these well because of the design of the tubes.
I'd replace the radiator if you know it is not the head gasket. To check for head gasket failure, there is a block tester that goes on top of the coolant tank. You can get it at NAPA stores. If the head gasket is OK, replace the radiator.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS