The cause is a faulty radiator. Mercedes radiators have square tubes in them that are not smooth inside. They will build up an area of blockage if the coolant isn't flushed aprox every 2-3 years. Once blocked, it has to be replaced. YOU ALSO SHOULD REPLACE THE THERMOSTAT WITH A NEW ONE WHEN YOU INSTALL THE RADIATOR.
We sell the radiators on the partshop!!
SEE BELOW

------------------
Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM