As Onkel suggests, first make sure you have a good cap. Also are you sure you aren't overfilling the reservoir - when the engine is cold the level should be barely above the "min" mark.

However you say the problem seems to arise only when you've done some high speed cruising (when the pump is flowing the most). If the engine isn't actually overheating yet the reservoir is still bubbling over, then it kind of sounds like something is restricting the flow, like a semi-clogged radiator core. If you remove both the upper and lower radiator hoses and feed water into the upper neck with a good high-volume garden hose, you can observe whether the lower neck is dumping the water out at a healthy rate or if wants to back up and overflow out of the upper neck. Don't seal the garden hose tightly to the upper neck as this will subject the radiator to full house pressure which is much higher than the normal cooling system operating pressure.