take note that there two different type of ECT installed. They are of different color for ease of identification, and have different cut-in temperature rating. make sure you have the correct cut-in temperature rating.
But from my experience, the culprit is normally the thermostat. If it is stucked up or delayed in opening, the radiator fan will
also turn on and operate at higher than normal temperature. It easier to replace the ECT and it also cost about 50% less than the thermostat.