OK, this is out on a limb but my Mustang used to also overheat when sitting still for a while (like in traffic, drive through, etc..) and it started doing this after I had to go 'off-road' one time to avoid rear-ending someone. I ripped the front plastic piece (black thing, air dam or whatever) and I thought that was causing it to overheat...
Long story short, I had to replace my intake manifold and when I did, I replaced the 180 degree thermostat that I had in my car with the factory one (210 or so..) and it does not overheat anymore. So if you have a 180 degree thermostat in there, try changing it back to stock. I've got a theory for why it was overheating but if you don't have a 180, it doesn't matter really ...
Good luck, hope that helps some.