might just be low on coolant, which means you might have a problem some where. if it is low, check around the thermostat housing for any signs of leakage, hoses , clamps and around the water pump. there is a weep hole on the waterpump that will leak if its bad, ect.. the plastic thermostat housings are known to crack.
05 V6 auto,
"The V6inator" torch red, base, amustangrocksleather padded leather armrest, shift boot and e-brake cover