The same thing happens in my truck. Battery light comes on at high RPMs. I did what Way Fast Witey suggested and replaced the brushes. One of them was significantly more warn than the other. When I looked at the rotor inside the alternator, I noticed one of the two copper slides that the brushes rub on was very slightly out of round. You can only tell if you spin the alternator with your finger running in the track. I believe the tiny bump at high RPMs causes the brush to vibrate enough to prevent a good connection. Replacing the brushes works because the springs press down harder when the brushes are new, but that's just a temporary fix.
Anyway, I might see if I can find a buddy with a lathe I can borrow. That should be a free fix. I'm not sure the rotor can be replaced independently, but if I was going to inspect a rebuilt rotor again, I would definitely take the brushes out and make sure those two slides are perfectly round. Lots of people have this problem and it may be possible that many alternators have this defect.