Conquest victory questions

The last two games, I was America, once with three other civs, France, England and Spain, and once with two others, France and Korea, about two hours in I was handed a Conquest defeat, both by France. The first game I was the largest and most advanced and there had been no wars, one city on a border converted and then it was over. On the next an hour in France declared war, took one border city, and that was it. I was as big or bigger than France, as advanced, and had a couple dozen Spearmen and Swordsmen nearly to the battle line at the border.

Both games were Warlord level, huge map, continents, AI at least aggressive. The second game had only Domination and Space Race victories turned on.

Can the AI get corrupted? Do I need to reinstall?

You activated the elimination option with the setting 1 city. That means that any civ losing 1 city is annihilated. Deaktivate the Option and you will be fine.