The thermostat is way down below the alternator under a plastic housing held on with two small bolts, under the PAS pump bracket (if you have PAS), follow the bottom hose out of the radiator and its at the end of that. Make sure you get a new O ring seal with it, use only oem parts for this important item.
If your thermostat was the problem it would tend to run hot or cold all the time. The temperature is supposed to rise a bit when you're not moving, so long as the fan cuts in and brings it down again that's quite normal. Not sure if the 16v has a 2 speed fan, it could be that the high speed is not working, but I wouldn't expect it to be needed this time of year.
