Almost always an RPM related loss of charging system voltage is from the rotor windings in the alternator. Especially when it is very consistant with RPM rather than from electrical load. As the rotors spins, one of the wires in the windings either opens or shorts from centrifugal force. Since you can't get the rotor by itself it still means you need to replace the alternator.
Back when alternators were routenly overhauled in Ford dealerships (many years ago), I was often the hero for being able to solve this problem when other technicians had failed. They must have fallen asleep during that part of the alternator rebuilding class.
I think that if you remove the alternator and have it tested at an autoparts store you will find that the alternator still acts up only at higher speed (if the tester spins that high).
