Where is the Thermostat located?

The 2.7 thermostat is located on the side of the block right above the A/C compressor. To get to it you need to remove the drive belt and then put the vehicle in the air and unbolt the compressor to move it out of the way to get to the thermostat housing bolts. With it out of the way you remove the housing and reverse the process to install the new one, make sure the purge valve is at the top.

If you mean that you can be driving down a highway for a while, then come into a town and need to go slower and the temperature goes up a bit, then when you get back on the open road it goes back to normal, that is normal. The reason is the airflow through the radiator is what removes the heat from the cooling system, so out in open air the speed of the car moves a lot of air through the radiator. When you get into traffic or stop and go driving like in town that airflow drops a lot and you become dependent on the cooling fans to move air. However, on most vehicles those fans turn on at a higher temperature than many think, most are at or over the boiling point of plain water. So, if you are driving on an open road and see a temperature of say 199-200 normally then you get into town, and it moves up to 210-220 or so, that is normal. If on the other hand it climbs too much higher like 240+ or higher then you will want to check the radiator for blockage. Dirt, bugs damage to it or the A/C condenser in front of it can lead to higher operating temperature.