Assuming all other things are good i.e. clean rad, good cap, good tstat, good CH gasket, etc...another thing to look for is the radiator lower hose. it could be collapsing (even w/o leaks). the reason is that the water pump creates negative pressure (vacuum) in the water jacket esp during high speed thus the highway overheat.

Plus, checking the cap operation:
1. Compress if it can be compressed (for relieving pressure)
2. Pull the inner spring if it can be pulled (for relieving vacuum)