Temp gauge or Coolant temp sensor?

[LeDevil]

Tonight after work I put in a new Fail safe 180 degree thermostat and Coolant temp sensor. Both of mine were pretty nasty. I ran the truck until it got warm (middle of the gauge) then I let it cool down and topped off the radiator I did this about 5 times until the level of coolant didn't go down any more. Well my temp gauge started working after the new Sensor was put in but now if the truck idles it will stay below the half way mark nice and cool but as soon as I drive it it will peg past the "H" like way past the "H" it just barrys the needle!
I'm hoping that the Sensor is just bad but if any of you have any ideas or opinions what would cause please let me know as this is my daily driver.

Thanks LeDevil