I just replaced the thermostat, fan clutch, water pump & radiator on my 1983 300D Turbo. I drove the car to work this morning & it ran at 80C on the interstate. When I got into town, the temp. went up to 100C at the lights. Outside temp. was 47F. Car runs smoothly. What could cause the temp to go up in traffic with all of the above NEW? I did notice when I replaced the thermostat, it had some brown rust colored film on the stat. I cannot see any oil in the coolant and no coolant in the oil. I am lost as what to do next. Could it be a leaking head gasket? PS I let the car idle for about 30 minutes with the cap off the reservoir tank (to get rid of possible air in the system) after replacing all the above.

Richard