Hyundai Repair: Troubleshooting code P0128, hyundai elantra, hyundai dealer

Question
My daughter has a 2001 Hyundai, Elantra. She was traveling to college and her radiator busted at the seam on top. I got a used radaitor and installed it. the Hyundai dealer said I should replace the thermostat as well(from the dealer), so I did. Got it back together and drove it for a day and the engine light came on. I had it read and it came back with a code P0128 temperature below thermostat regulating temp. The auto store reading it said I should replace the temp sensor, so I did (from the dealer). My daughter drove it for about a week and the light came on again. I had it read and got the same reading. She also noticed that when ever she stops that the heat stops coming out of the heater and the air goes cold. When she starts to move the air heats up again. Any help would be appreciated.

Answer
only thing i can think of is maybe the water pump is not pumping right maybe slow have the flow check ony thing i can think can stop heat from comming out is no flow to the heater core an when you start driving it starts passing coolent through it again this is only a thought so have ti check sorry i cant be much more help