Good point - thanks for the update. And it shows how monitoring the digital temp gauge readout helps diagnose the cause of the problem: If the temp rises in a linear fashion to the 170-180 deg range then cuts out to -- --, and a code p0128 is thrown as a CEL code, the problem is most likely the thermostat.

However if the temperature display jumps up and down erratically the cause points to the temperature sensor or it's connection.