Temp sensor resistance is inversely related to temperature, that is, it has less resistance the hotter it gets. Sounds like you may have corrosion going on in the socket. Try plugging in and removing the plug a couple of times and see if that helps. Probably not the wiring beyond the plug.
You can test the sensor with an ohm meter and a pan of boiling water: you should have resistance of around 3500 ohms at room temperature and a resistance of around 185 ohms in a boiling water bath.
If you do not know how to use an ohm meter, let me know and I can walk you through it....but check the socket first.