Hey navman, mine had the exact same issue as yours. Fine at low speeds, overheats on highway. Changed thermostat, radiator cap etc. Was fine for a bit but started overheating again on long hills. Frustrated the hell out of me, lol. Ended up having the radiator cores rodded and the bloke at natrad said it was 75% blocked! Has been perfect since then. It's a V6 though, but i don't think it matters cause we're talking about the radiator not the engine? So if it happens again maybe give that a go.