2001 3.5 LHS have had the car two years, and the car's gauge always shows car running slightly below 1/2, once up to operating temperature. This is winter/summer with AC system functioning, whatever. I noted the gauge showing slightly high on coming home from work one evening, and the cooling fans on high. Not usual. The next day, in slow gridlocked traffic the heat was goinng up, so I shut off the AC. It went back to near normal. I parked the car asap and opened the hood to see the coolant pressure bottle had a facture and a small stream of coolant was evident. Added as much water as I could for trip home. the car got close to the top of the gauge before I stopped and called AAA. Replaced the pressure bottle and filled the system as best I could. The level in the bottle has been in the min/mas range. Via the gauge the car runs a tad warmer than before, the gauge will go up maybe to the upper 2/3 for a bit and then drop down to slightly below the middle. I can hear the fans running on high when the gauge registers this way. I am wondering if the thermostat is going? I have a FSM, and this does not look like a hard job. I like this car, I do not like working on it. I am too accustomed to 50's and 60's Imperials. I know this is long, but wanted to give as much information as I thought might be useful. I have always gotten such great adivse here, I will thank folks even before they answer.