I believe that Gowron was perfectly in character throughout, and yes, he was consistent. He was a politician through-and-through. As is true for many (if not most?) of those who pursue social/political status in real life, the instinct to defend that status once acquired was stronger than his other desires or personal qualities. To protect his position was his primary goal. He could certainly be reasonable, he could certainly be honourable, but always the most important thing to him was the pursuit and maintenance of political power. I don't even say that in a particularly judgemental way, to be honest, it was just the way Gowron worked. In Klingon culture, where weakness of any kind is an invite to attack, he was probably even more committed to playing the political game than he would have been elsewhere. Martok was becoming a threat to Gowron, so Gowron's instinct was to sabotage the rival by any means. Not a smart move or a wise one, to be sure, but sadly expected for both Gowron personally and - as Dax notes - for the Empire.