Charging issue

On my 2008 Chevy suburban while driving for approximately one hour my
voltmeter will drop to 13 V occasionally it will come back up to over 14 and
then drop back down to near 13 I stopped checked with voltmeter and it was
charging at 12.9 to 13 V I replace the alternator and about one hour  after
driving same issue appeared I drove for about an hour then stopped checked
voltage again while running and it was 12 9 to 13 V if I turn on all of my lights
and air-conditioner full blast it'll jump to 13.2 any suggestions