I don't know what happened with George. In the series, he was believable and did what few character scenes he got really well. But during the films, his style changed. Maybe it was all the convention appearances. He developed a pompous, orotund speaking style that younger Sulu never showed. He started sounding increasingly phony, as if in love with the sound of his own voice.
That's it; I'd say you put your finger on it. Having been a bloviator in local politics and an eagerly applauded speaker at ST conventions, Takei infused film-Sulu with a level of vanity and pomposity that even Shatner himself couldn't carry off. That's why film-Sulu seemed mannered and artificial.
I agree with you both. In the series, he seemed to be trying to play, oddly enough, the 'average joe American' even though the show was in the future, of course. No attitude, no airs, just a competent officer who happened to be asian.
Then during the movies, he became more and more poumpous and affected.
It's like he watched too many Kurosawa movies and decided that he should be this 23rd century samurai. The ego boost he got from the fans and perhaps him thinking too much about "who is this man?", made him forget who the character was in the series.
You got to hand it to Koenig, despite being stuck with the accent and the silly way he was written in the series sometimes--he just went with it in the movies, never trying to reinvent Chekov.