I changed the thermostat, coolant sensor and coolant,cleared the code p0128 and the CEL came back on after about 100 miles. no other code is showing up. What other problem could this be

Even cooling systems can get a bad case of gas...and need to be fart....LOL! Agree with #1

I did bleed the coolant system about 2 weeks ago cleared the code and the cel came back. The engine temperature is rising and stays slightly below the center line as it always has. No leaks found.