on the highway you have the most air flow & the most water flow (at least the impeller is turning faster) so their is faster water flow if their ain't an issue which I'm thinking their is, so when you slow down & their is less air flow and less water flow & it runs cooler then that tells me that at the higher speed when the eng is producing more heat that the air flow is OK & that something (the radiator) is restricting the water flow.