Is the radiator stopped up? At low speeds it would build up pressure and maybe expand the hoses but at higher speeds maybe the pressure is enough to force the water through the radiator ? Just a guess, could be a number of things, best bet would be take to a good radiator shop and get it tested.
I vote for a clog/blockage in the cooling system, causing a hot spot to boil, and pressure. The water is not circulating at that location and the boiling causes the pressure.
