It's not failing at a high voltage level. That's the proper voltage if it is charging or if there is a load on the system. You have the same thing that I had with the SD. The alternator output would fail, and the battery would die. No alternator indicator light, however. Charge the battery and the alternator will then seem to work for awhile (maybe three of four days). Then the process repeats iteself. It must be the brushes losing contact periodically due to vibration. Time for an overhaul.

But, I agree with willrev on the use of a Bosch rebuild. The first time I had the alternator rebuilt by a local shop, they installed aftermarket parts and they lasted two years ($75. gone). Take it back to the same shop and they imply that that the aftermarket parts are just as good as the Bosch and that nobody really knows when they will fail. Just luck of the draw. (another $75. gone). Never again.