2005 Honda Civic Temperature Gauge

Hello: I just got my radiator replace about a week ago. The temperature sensor was also replace. All fluid were change. But now the Temperature gauge is going crazy. When am driving through freeway or highways the gauge shows that its fine, but when I start slowing down or actually stop. Thats when the needle goes up to the Red (H) and its telling me its overheating. I pull over and the radiator is fine am not overheating. Then once I start going and don't stop the needle goes back down. What do you think it is? HELP!