Is the cooling fan operating properly - as in, does it come on at like 180-190° or so, or does it not come on until the gauge reading indicates a near overheating condition? Also verify that it comes on when a/c or defroster is on; it should stay on constantly when the a/c compressor is running.
The second one is what it sounds like may be happening.
2001 GT Convertible
8 cylinders, 5 gears, red.
6 cylinders, 3 gears, also red.
Remember my motto kids: Be bitter, stay angry and blame everyone. It's on my fridge magnet!