Thermostat?

Have a 2002 Mercury Sable with approx. 55,000 miles. Last night the heater suddenly blew cold and the engine temp went well above normal, but not quite into the "Hot" zone. After a minute, the heater blew warm air again the the engine temp went back to normal. The car did this one more time a few minutes later, has not repeated.

I assume this may be a sticky thermostat, but would like any input, also would help to know where the thermostat is on this model?

Thanks,
Terry