Posted for "Neal Garvin" <nrgarvin@comcast.net>:

 Check all the connections especially the field connection on the alternator
 and regulator. If it is a "fast-on" or similar quick connect plug type, they
 can loose their "grip" and the resistance can build causing the alternator
 output voltage to drop which the regulator senses and corrects by adding
 more voltage (or is it current?) to the field. The poor connection can arc
 or vibration can drop the resistance for a moment and the alternator will
 pump out an overvoltage.
 
 I had this problem and it took me 2 months to finally identify it (with help
 from Bill Bainbridge at B&C).
 
 Are you using a B&B Alternator? If so, you need all three connectors in the
 male plug housing to stabilize it. You can interconnect 2 of them to utilize
 both field connection tabs in the alternator for redundancy.