most common cause of overheating during highway driving etc is the radiator being partially blocked. Over the years sediment builds in the cooling system and finds it way to the radiator where it builds up from the bottom up, partially blocking it and reducing its effectiveness. Flushing wont get rid of this type of sediment, the radiator needs to be removed, taken to a radiator shop and they will professionally clean it. Or replace it if your extreme or want an alloy one etc.

Easiest way to tell is get the car to operating temp and (depending where your fans are!!!!watch out they can go even if the car is turned off!!!) try to get your hand on the radiator core fins and feel if there is a great temperature difference between the top of the radiator and the bottom, the top will prob feel to hot to hold your hand there for over a second or so, but if blocked you should be able to hold you hand on the bottom for atleast a few seconds or longer if really blocked.

Like i said watch out for the fans. I'm not saying this is definitely your problem, just saying that at our workshop this is the most common cause of overheating at high speeds