Son got home and woke me up at 10:00pm last night telling me of a radiator hissing or spewing, (his term, not mine) and said the heat gauge in his car would peg to the red, stay there for a few seconds and then drop back down to cold. He said the car would never warm up. I checked under the car on the way out the door this morning and there are no leaks.
Tonight when I get home I'll check fluid levels, make sure the heater core isn't leaking and then assume that I need to go buy and replace the thermostat.

Can you that are auto knowledgeable confirm I'm on the right track here? If not the thermostat, what else would cause the car to act like that? FYI: '98 Ford Mustang 3.8 auto.