Do you checked it with the cheat editor? I heard that it also depents on the goods settings I never test. But you can play on difficuilt without any trouble.

I made a little privatemod including CityMod2, DiploMod, some parts of FixMod and some private modifications. There I saw the first time that one AI nearly conquered another AI, but than I came...

I had only some trouble with pollution, maybe I shouldn't mod the pollution and growth stuff.

Even in my last Med Pack game the AI didn't declare war on me although I haven't the best relationship to them. I wonder if I started really a game on impossible. Therefore you can also try it on impossible.
-Martin