Take the thermostat out and see if it goes back to being too cool. If it does, it has to be the thermostat and no amount of other measures are going to do anything. The flat part of the thermostat goes up. The top radiator hose will not show the temperature of the engine coolant if the thermostat doesn't open; the heater hoses would be a better indicator when you are looking for a place for a temperature probe.
