I had a similar problem last week, except that I didn't change the radiator, the temp gauge just stopped working, turned out to be the connector to the sensor, it was a bit loose and corroded/dirty. I gave it a clean and sand and it worked right away. You probably know already, but for anyone like me who didn't know which one it was, it's the sensor closest to the engine with a single wire coming from it, on the housing which connects to the upper radiator hose.