A couple of thoughts... be sure the hoses are firm to the feel and not spongy. If the bottom hose is soft, it may be collapsing at highway speed. If the pump isn't leaking, it may be okay as I haven't personally seen an impeller fail without the seal leaking as well.
With the engine cold, or just slightly warm, remove the radiator cap and verify water circulation with the engine at idle. This would indicate the pump is moving water at least at idle. If the radiator takes a big burp while the cap is off, it may indicate there was a vapor lock in the cooling system.
Lastly, if it is cool at idle but hot at speed, it may indicate the radiator is not flowing properly and may need some attention.
Good luck,
Gary