One simple solution that should work is incremental difficulty levels.
After x number of turns, the AI difficulty level goes up by one level!
You could tweak it to delay the level up if the AI power rating is x amount higher than the player. So whatever happens, the leveling mechanism is going to try to keep the game interesting and avoid it becoming too hard or too easy.
Another option which would be fun.... AI discovery of per-curser fleets!
It will be only available for the most powerful AI the player is at war with and only if its power rating goes below the player. The more the power rating goes down, the more chance its got of getting the fleet. How powerful this fleet will be is determined by the player technology level, it should be significantly more powerful than what the player can produce..... that would be FUN!