I play on huge maps, and everytime 3 players (AI or Human, doesn't matter) make an alliance against a CPU, it commits suicide after 1 or 2 turns. The only city, which remains from it's empire is the capitol. It's annoying, since it makes the map pretty empty (because the AI never colonise the ruins of the once great empires), and the victory too easy. This happens even with the most powerful AI. I used Civ3MultiTool, to correct one of my accidently screwed up deal, so maybe it's the source of the problem, but it would be strange.