You might also want to monitor your oil temps.
As for diagnosing, I mean you've changed out the sensors already and thermostat, maybe there's a blockage in the radiator that allows just enough fluid to pass to make it seem like the system is working? Last things to check are really how much flows through the rad and if it flows well, is the water pump actually working properly. If ISTA kicked on the fan that shows the relay is good.