If you have a head gasket leak between a cylinder and the water jacket, hot combustion gases will superheat the water on the power stroke. The faster you go, the more often the transfers of hot gas will happen. A compression test may detect a head gasket problem, but could be masked by worn rings or valves. Another tell-tale sign is air bubbles in the coolant when the engine is idling.
An partially obstructed radiator is another likely cause.
Idle the engine with the radiator cap off (wear safety glasses), and see if you can detect an obvious increase in flow when the thermostat opens (check for air bubbles at the same time). I agree with those who suggested a radiator flush. I've used household Drain-o and lukewarm water, and it works fine. When flushing out the Drain-o mix, if you see lots of brown liquid, chances are the capacity of the radiator was diminished.
An partially obstructed radiator is another likely cause.
Idle the engine with the radiator cap off (wear safety glasses), and see if you can detect an obvious increase in flow when the thermostat opens (check for air bubbles at the same time). I agree with those who suggested a radiator flush. I've used household Drain-o and lukewarm water, and it works fine. When flushing out the Drain-o mix, if you see lots of brown liquid, chances are the capacity of the radiator was diminished.