@ Saboteur Wow, that's really interesting. I've had similar experiences. I had one game at Prince, where I was way ahead in the coming UN Vote, and suddenly other AIs start dumping gold on CSs, while Askia started murdering CSs. I've had a game at King where I dominated my continent and had 9 solid CS allies, but Russia was building space ship parts in the 1980s so I figured I was toast. I bulbed to Globalization, and won an easy diplo victory (also not satisfying). Russia should have won this game, but they apparently went on leave. The AIs are ALL OVER the place in this game. I've seen Siam pull off some master tactics that shocked me, while I've seen other AIs go brain dead.
If you consider that the AIs have no victory scenario planned, it makes you really rethink the game. Civ 5 is so centered around specific victory scenarios for high level victories for the ambitious player, yet if the AIs can't aspire to these same victory stategies, it makes me question the quality of the AI in this game.
I been beaten multiple times in the science race, and diplomacy in Civ 3, in hard fought games at Monarch and above.