2011 Forester 2.5X Automatic
Interesting nobody has suggested the radiator. My first impression was the radiator is compromised to some degree. When driving around town you are not putting a significant load on the engine; the present condition of the radiator can handle the heat load. When you run out on the interstate for multiple hours at speed, the increased airflow through the radiator compensates for the inability of the radiator to shed the heat load. And I suspect the engine is running a bit hotter than normal but not hot enough to turn on the dash board idiot light. When you slow down after a good run on the interstate, the airflow decreases but the engine is still quite hot, as is the coolant, and the radiator at this point cannot accommodate the added heat load with the reduced air flow. The coolant temperature spikes and you have an overheat condition.