I have Chevrolet Lumina S (V6) 2005 model. It has problem since one month. The problem is that the temperature is no stable and it is fluctuating within the normal range. It sometimes reaches the half of temperature gauge (indicator) in the dashboard. I have changed the thermostats but still the problem is there. I can hear the cooling fans running. This problem is specially there with the AC on but I am not sure if it is not their when i drive the car with the AC off. The coolant fluid level is OK and the engine oil level is also OK. The temperature should be usually stable at one level with the AC on.