[jakeru]

The following is general, non-boxster specific knowledge, but maybe helpful for you:
When automotive alternator brushes wear down so far that they stop making contact with the commutator, this will cause charging failure and voltage drop, potentially leaving you unable to start the car, or stranded on the side of the road, and various other accompanying symptoms. In this scenario, I've experienced the alternator charging "come back to life" (measured by > 14v) after letting the motor cool down completely, but it won't be a stable fix - as the brushes are just barely making contact and will continue to arc & wear, the alternator will have another charging failure soon and it will get progressively more common.

To avoid this sort of failure, when the alternator has very high miles e.g. around 200k or so, just preventively replacing the brushes is not a bad idea. If you are experiencing a charging issue and miles are > 100k, consider at least pulling the brushes and inspecting for adequate length (if not a hassle), or if it is a hassle to get to the brushes, just replace them with new (and if the problem end up being something else in the process, this will at least you ahead on preventive maintenance in the process.). I believe in the 986 alternator design, the brushes are part of the voltage regulator assembly.

You can also replace the alternator with a "rebuilt" one, or have yours rebuilt, which will include getting new sealed bearings, and lathed down commutator segments, in addition to new brushes.