stormy, suggest installing a new Wahler thermostat. It's likely the existing one doesn't open completely, causing the engine temperature to rise. I have had the same symptoms, only in the Winter months. Replacing the thermostat resolved the problem. Be sure to orient the new thermostat with the ball toward the fire wall.

Fill the engine block with coolant (you should see coolant just below the thermostat) and fill the radiator with coolant using a clear piece of tubing attached to the upper radiator hose connection. After reassembly get the engine to operating temperature and check the coolant level in the expansion tank. If the coolant level is low wait for the engine to cool and add coolant to the full line. Use a 50/50 mix of either genuine MB coolant, or Zerex G-05, and water.

A defective thermostat restricts coolant flow. Unless you see evidence of a leaking water pump seal (coolant coming out the weep hole, or white residue from the weep hole) there should be no need to replace the water pump.

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Fred Hoelzle