Temperature sensor not working properly

Take a look at the attachments for how this works and the wiring diagram. Basically if we can assume the sensor is okay, then the only thing left is the wiring or the cluster itself.

Here is what I would suggest, monitor the voltage from the sensor by back probing the green wire and let the engine run. This way we know what the sensor is telling the cluster. Then we can do the same test at the back of the cluster. If we are getting steady voltage change then the sensor is working properly. If you are getting a good signal at the cluster then we know the cluster has an issue on the circuit board. If not, then you need to run a new wire as there is an issue in the wire.

The fact that this seems to work okay at the beginning and then shoots up tells me it is the cluster. Let me know what you find or if you have questions. Thanks