Try installing a different battery temporarily- - - -quickest way to eliminate that as the source of the problem. There's also a possibility of a diode and/or internal regulator problem with the alternator that only happens when the unit is warmed up. If the problem persists with a known good battery, it's time to dig deep into the alternator/regulator circuits- - - - -maybe use a heat gun to stress the alternator and see if operating temp has an effect on the output.
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!