I think it sounds like a faulty sender. I had one do exactly the same thing, but when I checked with a thermometer in the radiator, it was just under 150 after running for 10 minutes from a cold start. Of course it's just a guess, but I would check the actual temp if you can just to be sure it is malfunctioning. Yes, that is the intake heater with the relay but that would not cause an engine overheat. Fan clutch, clogged radiator, worn water pump vanes, or a thermostat stuck closed will all cause an overheat condition. But I would start with a replacement sender as they are cheap and can be defective. Let us know how you make out.