I have a feeling the temp sensor is bad. It's intermittent. Start the car and the temp gauge goes to max. Turn it off, wait a sec, turn it back on and it's fine. Hoping it's the gauge and not the wires because I'm pretty sure find a fault in the wire is going to be a much bigger pain than replacing the sensor.

So, where is the sensor? Pictures would be a big help.

 

It’s the sensor that’s mounted onto the head next to the thermostat. Pull the plug and see if its wet, if so then it’s a faulty sensor.