Do you buy a new radiator or a used one? With a bigger radiator, it doesn't make sense to have it over heat at 60 MPH. What RPM are you turning at 60 MPH? If you got a used radiator, I would put it, take it to a radiator shop ane get it cleaned out.
Is the motor stock? It would make sense if you were pulling a trail, up hill, in a head wind, in 90 degree weather, I'd say try a lower gear. But just by itself, there is something not right.