95 Mercury Sable overheating?

My commute to work involves me driving about 10 minutes in the city, 15 minutes on an urban highway, then 5 minutes in heavy suburban traffic. On the way home, I have noticed that when I get off of the highway and am sitting at a red light, my thermostat tells me that my engine is getting hotter and hotter. As I drive, the temperature drops a bit, but it is still sitting at a temp that is higher than normal. My coolant level is normal, my radiator is almost 1 year old (it was replaced last summer). I am curious as to what could be wrong. Why does this not happen on my way to work? I do take night classes and the same thing happens when I drive to class (about 15 minutes away, all city streets). I also noticed that this is never a problem when the outside temperature is cooler (below 70 degrees).

You probably just need to replace the thermostat, but you should also check the function of the cooling fan. As to why it doesn’t (or hasn’t yet) done it one the way in - could just be coincidence - sometimes the thermostat sticks and sometimes it doesn’t?