I had a similar sympton on my car recently. I thought for sure it was the brushes. After all, the car has over 177K miles on the original alternator. However, it turned out that the insulation on the thin red wire to the alternator had deteriorated to the point where the electrical connection on the alternator was intermittent.

I thought that my car had somehow escaped the notorious wiring harness deterioration problem. No such luck. As a temporary fix, I coated the wire with black silicon and am now looking for a replacement harness.

Good luck,

Leon