IIRC, some of the Delco regulators had a temperature-compensating system built into them, but it was to increase the voltage slightly under hot operating temperatures. It's been close to 30 years since I did much work with generator systems on a regular basis, so I'd have to look up the specifics of your system to make an educated guess what's going on. Regulator points open and close several times a second under normal operating conditions, and they have a slight wiping action to help keep the contact surfaces clean. If any roughness develops on the contacts, it's possible for them to stick and cause the voltage to go up until the sticking condition corrects itself.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!