Since you have done all of the obvious fixes, consider this. if its overheating on the freeway, that could indicate a potential lack of airflow through the radiator. My N/A had a similar problem a few years back. It ended up being debris in the condenser and radiator fins. At low speeds the engine creates less heat, and therefore requires less airflow to maintain temp. At higher speeds more heat has to be dissipated. I would suggest pulling and backwards flush the outside of the radiator and condenser. This can easily be done with a can of foamy engine cleaner and a garden hose. I was amazed how much crap was in my condenser. Years of road dirt accumulation had plugged it up badly. There was a lot that went through it and got caught in the radiator. I also got a condenser comb and straightened the fins, as they had a lot of rock damage. I have not had a temp issue since. Hope this helps.
