[concobra]

I think you have already hit on a key point to help diagnose the problem. That is your comment that the car ran cooler last year and that you had not changed anything means the current cooling sytem is up to the job. Just something has changed. I suggest diagnosing the problem before throwing parts at it.

1. Check your current gauge. Put another gauge on and compare.
2. Remove your current thermostat and place it in water on the stove with a thermometer and monitor the opening point. And that it opens fully.
3. Get and infrared thermometer gun. Check the temp at the themostat housing. If the thermostat is OK the temp should read within 10 degrees. Check the inlet of the radiator, and the outlet. The outlet temp should be 25 degrees cooler than the inlet temp. If not your radiator is not transfering heat well. Could be clogged, your fan is not moving enough air (low amperage), your moving water too fast through the system.
4. If you blew a head gasket you will have a pressure build up in the cooling system.

Now some answers to the above checks.
1. Since it heats up both when you are driving at speed and driving in traffic. It is probably not your electric fan. Otherwise it would only overheat when in traffic.
2. It's probably not you thermostat. Otherwise it would way overheat without any control.
3. Since the temperature does stabilize (if I read your comments correctly) It appears to be heat transfer out of the radiator or water movement through the system. This may sound stupid but check and make sure your water pump drive belt is tight. Pull your radiator and get it cleaned out. Sediment can build up quickly if a car sits for a long peroid of time without running.


These are many possibles but please keep in mind it was fine last year. So your solution is not to buy new parts until you find what has changed.