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

 Check the push-n connectors on the back of the alternator and all other
 field connections including voltage regulator. The slightest bit of
 looseness results in arcing and oxidation of the contacting surfaces causing
 voltage drop which the regulator senses so it increases the field voltage to
 up the alternator output. Eventually at the peak of one of the up/down
 swings of the voltage, it will spike causing the field breaker to trip via
 the crowbar OV protection circuit.