John;

Good diagnosis and advice. The only thing I would add is that the battery itself could have a problem (open cell or internal connection maybe) in which case it would not clamp the voltage either.

Thad;

FIRST! Check and clean all electrical connections around bat., starter, reg., and alt...if problem occurs again after that, temporarilly connect a voltmeter across (clean) battery terminals, so you can verify its voltage next time the system voltage takes off...if battery voltage takes off, it's suspect...if not, connections and/or charging system is.

Good Hunting!