The temperature gauge on the dashboard is jumping around after the car warms up. It rapidly goes from cold to normal in an unpredictable pattern. It never goes over the middle. I have had both the engine and the radiator replaced in the past year!
You probably have a bad coolant temp sensor. They sometimes do go bad. It should be located near the thermostat housing. Be careful, because there are two, one for the gauge and one for the computer. You need the one for the gauge. It should have a single wire. The one for the computer usually has two.
I agree with BK. It’s more than likely your temperature sensor for your gauge.
The trouble may also just be due to a bad connection at the sensor. Make sure the connection is clean and making a snug connection.