2003 Chevy Impala Thermostat reads low and getting P0128 code

Heater problem
2003 Chevy Impala V6 Front Wheel Drive Automatic 85623 miles

My car is sending a P0128 message that the car is not operating at the proper temperature and is showing low on the temp gauge. I have checked the coolant level and it is fine. The reservoir has fluid and there is no air in the system as I checked the bleeder valve. THe fluid is hot and I get heat in the car fairly quickly. The coolant looks clean. No leaks or funny smells. I clear the message and it will eventually come back. Some times if the car is sitting and idling it will reach the normal temp range while sitting but then will cool down when you start to move again.