After Robert McCullough was fired at the end of Season 3 (either because of his onset manipulations or because Lorimar didn't want to pay both McCullough and Earl Hamner) the show was never the same. I found the first two-thirds of Season 4 to be watchable enough until CBS demanded the year's main storyline (the nazi treasure plot) be ceased immediately, 10 episodes before the end of the season.
To me, that was when FC derailed.
Season 5 was drab and cluttered. Freilich's Season 6 re-energized the show despite some goofy "comedy" thrown in, while his Season 7 became too obnoxiously frenetic. FC tried to go straight again for Season 8, but its mojo had already been stolen. And Season 9 was like a different program.