leaking on top of thermostat housing 2007 mustang

When things are cold they contract. When they are heated up they expand. Sounds like the thermostat housing is warped slightly and when it's cold and contracted leaks. As the engine warms up and everything expands the leak seals up. If you haven't already done so, try some nonhardning blue Form A Gasket. Put a liberal amount on the thermostat housing when the engine is cold. This type of sealant will remain flexible and keep things sealed up. Hope that helps! Jim