mk1 1600 petrol focus

had a suspected overheat with the engine going into 2cyl mode

checked it over inc water pump , stat, flush rad , new cap,cyl heat sensor appears to run at normal temps , no water loss , runs fine , no oil in water

the gauge swings randomly into hot and back again, engine still feels normal to the hand , live data and the dash secret menu says its 130-140

i can find a few threads with similar faults but no one posts with what was wrong , all i can think of is new wires for the cht sensor and someone suggested ecu??

help please