1992 300TE 164,000 miles. Overheats when idling with the A/C on. Goes from 85C to 110C in about 4 minutes. Rev the engine and the temperature goes down. Car drives down the road fine. Temperature is in a normal range when driven down the freeway 80 to 90 degrees C.
Checks done so far
1. The radiator is about 1 year old and a an OEM behr.
2. Just for kicks I looked for cold spots on the radiator and found none.
3. Thermostat opens, pulled when hot and it opens up. Approximately 1/2 to 3/4 of one inch open. Thermostat also looks new.
4. Engine fan clutch passes the rag test. With the engine hot, holding a rag to the engine fan I cannot stop the fan from moving when the car is idling.
5. One auxiliary fan was replaced and the crud that builds up behind the fan shroud in front of the A/C condenser has been cleaned out.
6. Both Auxiliary fans are working when the car is overheating.
7. Radiator cap holds 18 PSI.
8. No oil in the radiator or water in the engine oil.
9. Water pump looks old or possibly original equipment.

Right now I want to replace the water pump for possible circulation problems but a head gasket also comes to mind? Can anyone think of something I am forgetting to check B-4 I replace the water pump?
Thanks John