I would still argue this could be your thermostat. If it were stuck partially open or fully open, at idle the lack of airflow through the radiator would allow the engine temp to rise. At highway speed, the radiator and a stuck thermostat could create an overly cooled engine.

Sure, engines tend to run cooler in the winter and hotter in the summer but over an extended period of driving at any reasonable air temperature, your thermostat should be able to get your engine to the correct operating temp (assuming you are keeping a relatively similar speed).

I'd change the thermostat and check your coolant safe temperature.