it sounds like your fan isn't pulling enough air at speed and becomes a block.
you can test this by turning off the fan while at speed, and see if the temp stays low.
if it still goes up then it is a flow restriction, which you would need to investigate, stuff like a collapsing hose at speed. you could stuff it into neutral at highway speed and see if it goes down or put a go pro into the engine compartment and watch the lower rad hose.
fan solution would be to get a better fan, or cut vent slats into any kind of shroud you have, flaps really that only open at speed,.