My 2004 ford freestar heats up to 3/4 after driving 15min then cools down

I would drive the car with a proffesional grade scan tool attached an see what the scan tool reads and plot the temperature in relation to the actual gauge readings. Is the engine overheating? Are you loosing water? Next I would suspect a restricted radiator core, the radiator fan isn't operating at the correct temperature or the head gasket may have a mild leak (to prove this have a chemical block test done its a cheap test to look for the presence of carbon monoxide in the cooling system.

Most common cause is uneven coolant flow. Possible radiator if thermosat operating. be sure coolant level is correct without any air.