While riding in her 2002 3.0 auto GT I noticed her temp guage would fluctuate between HOT and COLD and her heat would somtimes blow hot and cold as well.The car does not overheat though and otherwise runs ok. Would this be a sensor,thermostat, or something else causing this?

 

sounds like its low on coolant. have u checked the level?? if it is low, 2 common things leak coolant on that car. radiator splits at upper hose area, and the necks on the engine for the radiator hoses deteriorate and also leak coolant. again very common.

 

That or it might be the thermostat or you might have a gasket that is not sealled all the way on your intake manafold I am having the same problem with my 03 spyder gt