Ford Repair: 98 windstar GL minivan cooling system, coolant leak, head gasket

Question
140,000 miles, 3.8 v6 engine
Temp gauage shot to HOT after traveling aprox 15 minutes on interstate @ 70 mph. AC started blowing hotair. Turned air off, slowed down to 55 and temp gauge kept flux. some but continued to go back to Hot. Turned heater on and gauge continued to stay hot. NO steam was coming out or no water in oil. Just changed and filled system with coolant a month ago. When arriving home, water pump was not leaking however even though coolant was present in reserve, the radiator was about 1 gallon low. As engine began to cool (being turned off) no coolant returned back to radiator. Causes? Fixes?

Answer
could have air ins system and it will have to be purged-sticking thermostat-clogged radiator leaking timing cover gasket is common on these-coolant leak somewhere else-bad head gasket
you need to have this checked before you drive very much. it don't take much to blow a head gasket on this engine.