Very interesting idea there, ColoradoBBQ - Dark Ages to even out the Golden Ages. In fact, that would be one way of balancing the game - civs near the bottom in score have high probabilities of entering Golden Ages, and civs that are right at the top of the the score table have an increased chance of entering a Dark Age.
Although I don't know whether the "Dark Age" should be quite as powerful as you suggest. I think it'd be more fair if it was sort of the opposite of a Golden Age - 1 less shield in all shield-producing squares, etc. Maybe a bit more unhappiness, but not TOO much (a Dark Age should just weaken a civ, not practically destroy it IMO). Less gold per turn, etc.
I wonder if there's any SIMPLE way of implementing this into the game?