Same thing was happening to my wifes car. We drove on the alternator for a year after it started. Just didn't rev it often. What I found was uneven wear on the shaft the brushs ride on. It wasn't round any more. I think once you get in the higher RPMS it jumps off. I had it turned down and replaced the brushs and regulator while I had it out. It isn't a process every one can do. Wires need to be unsoldered and then I turned it on a lathe. It would probably be good to replace it. The brushes were fairly worn at 80k miles.
