On startup, my system voltage will sometimes spike to as much as 16 volts, then settling back to the neighborhood of 13 - 13.5 volts near idle. While I realize there's some failure withing the alternator/regulator, I'm wondering just what that failure might be.

It's similar to a problem I had last year where the system voltage would spike without warning and all the lights in the car would get very bright all of a sudden and the heater/AC motor would run really fast. Turned out the screws securing the regulator to my rebuilt alternator had been stripped and worked loose, leaving the regulator without good contact to the alternator housing.