My advice, based on past experience with several cars, is have the radiator taken apart and rodded. or replace it. You may have been using 'green' stuff rather than MBZ cooolant. There IS a difference. Works fine 75% of time, then water chemistry changes with age and BLAM--insoluble white hard deposits starting at the cooler ends. Quickly plugs tubes solid (as it plugs less flow, cools the slow flow more, more deposits, etc, etc.
If replacing, get the thickest core you can, desert service.
Also do not overlook the water pump. Use ONLY MBZ dealer supplied pump (many rebuilts are available from dealers). After market rebuilders do not hold the close tolerances required for the pump to work efficently in hot slow conditions. Also run less glycol in th ecoolant mix (70% water, 30% glycol) will control corrosion, but cool better.