If your vehicle is showing vehicle overheats only when driving at highway speeds or uphill, here are the most likely causes and the parts you'll need to fix each one.
A low coolant level reduces the system's overall thermal capacity. It might be enough to cool the engine around town, but cannot absorb the massive heat generated during highway driving or hill climbing.
At highway speeds, the vehicle relies on ram air. If the plastic splash shields under the bumper were left off after a recent service, air bypasses the radiator. Alternatively, bugs and debris can block the fins.
Internal mineral buildup or sludge restricts coolant flow. The system can handle low-speed driving, but cannot flow enough coolant to keep up with high engine loads.
A weak cap fails to hold system pressure, which lowers the coolant's boiling point. Under heavy load, the coolant boils, turns to steam, and escapes into the overflow.
If the thermostat only opens halfway, it restricts the maximum coolant flow needed when the engine is working its hardest at highway speeds.
At high RPMs, the water pump creates strong suction. If the lower hose is weak or missing its internal anti-collapse spring, it can suck shut and block coolant flow entirely.
The water pump blades can corrode away over time. It circulates enough coolant for idling, but cannot move enough volume to cool the engine at highway speeds.
Under heavy load (like driving uphill), extreme cylinder pressures can force superheated combustion gases past a weak head gasket into the cooling system, causing rapid overheating.