Overheating wowes that make no sense - help!!!

I have had the waterpump with a impeller that was loose on the shaft causing the shaft to freewheel during speed (impeller moving less or not at all) but would Idle fine. Took about a year to figure that one out in a 66 Valiant. The up side of this is I gave the owner $16 bucks for the car. Good luck and please let us know your findings.