any of the suggestions stl made would be reasons for coolant loss and high temp. once you loose enough coolant, the gauge will not read right. the thing with heads is, it could have been something as simple as the radiator cap or a hose leak, but then running it hot with low coolant could blow a head gasket, or worst case crack a head. if you have a blown head gasket, you should see foamy looking stuff on the dipstick. a compression test might help checking the head gasket. there is a weep hole on the water pump to take a good look at. if they go bad, most often will show signs of a leak around the hole.
05 V6 auto,
"The V6inator" torch red, base, amustangrocksleather padded leather armrest, shift boot and e-brake cover