Engine overheating

Hi everyone, I have a question that I might know the answer to already, but just want to confirm it with your thoughts and answers... I have a 2002 Mustang GT that I bought brand new, and this summer when it was hot out, driving with my A/C on and in stop and go city traffic, the thermostat goes to about 3/4 the way over and the A/C turns warm. Normally the guage is exactly half way. Up until this summer I could use the A/C in city traffic and the guage would never go past the half way mark. Now the engine over heats, and it did today with the temperature at only 77. If I turn off the A/C, the guage goes down pretty quickly, especially when moving. My fan runs like it should, so it's not a fan issue. Is it possible, and I'm guessing that it is, that my thermostat isn't opening up all the way? I've heard of thermostats sticking totally open or closed, but is it possible that it just isn't opening all the way? It won't overheat going down the highway with the A/C on, but in stop and go traffic, it overheats.


Thanks...