So after my post on the topic I thought I'd have fun and set up a quick game on a small map with India, China and Persia as some of the Civs to test my prejudices about these three Civs. Unfortunately I was on another continent to India and China so couldn't see what they were doing. But it appears in this game, they didn't really fall into their respective habits of tech slump and tech sprint (although India may have squandered a strong start according to the graph). Persia, however, presented an absolutely beautiful example of what Persia does in about 50-60% of my games. I was his unfortunate neighbour Sumeria, and this is a brief history of an example of my experience of the crazy Persian Industrial era power spike. Here is the map at the end of the turn after Xerxes had declared war on me and frankly beaten me (taking Akshak to the north and a city beginning with Z to the south in the first of what would likely be many of my cities before he would ever speak to me).
And here is the power graph showing Persia heading for lift off in the Industrial era (you can just see the Persian graph starting to jutt out to the left as mine falls by the equivalent amount having lost two cities). Note that his score is completely ordinary. A thoroughly middle of the pack Civ for the ancient and medieval eras and before his Industrial era aggression his score was only 4th out of the 6 Civs on our continent. Any other Civ in the game and I would not be concerned about them as we approach the Industrial. But for this one I had my eyes on him given my recent posts on the subject.
Overview of a typical Persian AI's game
Ancient era - Persia borders me and a somehow puny Scandinavia that botches its expansion phase (maybe there were barbarians in his neighbourhood?). Persia is stuck in the north of the eastern continent, with my mighty self as Sumeria, keeping him bottled in and on good terms. Persia behaves like a typical stupid Civ 3 AI in this era. He doesn't attack the puny Scandinavians that have only 4 cities. No, instead he sends his Immortals to fight the advanced Byzantines in the south east of the continent. Byzantine is my preferred tech trading partner as the French and Mayans scare me a little. Xerxes has left it too late , Byzantine has pikemen and the war quickly fizzles out into a stalemate.
Summary: Persia has squandered a really promising opening with a weak neighbour and sank to the lower middle of the pack amongst the AIs by picking a stupid long distance war and effectively wasting his insanely powerful UU (another trait of his).
Medieval era - Byzantines and myself are firm friends adn MPP after France attacks me. It delays our teching a little bit, but we are preferred trading partners and I trade with them every tech I get. We are both scientific and trading techs at pretty much full tilt. I am absolutely prioritising Education and Universities for culture effect as I have some questionable borders that are at risk of flipping. However, Persia, despite still being limited to up in the north of the continent is notably out-teching our combiined efforts throughout the Medieval era. Byzantine drops out of the war with France so I MPP with Maya to rebuff the mighty French (and end up picking off a couple of French cities, one of which was originally Byzantine). The point here is Persia attacks the Byzantines again (again ignoring the low hanging fruit of Scandinavia) and his knights have no greater luck against the Byzantine pikemen and musketeers. It is a fairly short war and Persia manages to maintain his tech lead despite again being a stupid Civ3 AI making poor military decisions. With the French aggression subdued, I dissuade further aggression from the Joan of Arc (and culture flips) by garrisoning the bulk of my military in my new French conquests. I am aware though that my game has become all about racing to Nationalism (with the Byzantines) to get Riflemen and draft, so that we can jointly repel the Persians, hopefully before he gets Cavalry.
Summary: Again, sub-optimum by Persia. He could have mauled the Scandinavians or even had a go at me as Sumeria whilst I am stuck in a war with France. He looks geographically locked in and contained. He is however, teching like a god despite being caught in a war and is outteching the combined efforts of 2x Scientific Civs of broadly comparable power and wealth.
Industrial era - the Byzantines and I combined cannot keep up with the Persian tech and he gets Cavalry about 15 turns before I could possibly get Riflemen. Nobody else in the game has cavalry. It is clear he can do me serious pain at this point of the game (due to the Deity level cost factor I have for the AI and the fact my military is outdated due to my focus on teching above all else). I am still trying to keep him sweet so have given him ROP for much of the game and thrown in gold per turn bonuses and the odd luxury. Unforunately he uses the ROP to sweep his cavalry down to the Byzantines and quickly chomps off half of her territory. Third time has proven a charm for Xerxes. I manage to let the ROP with him lapse to reduce his military effort against Byzantine and he sues for peace with her and then boats over some possible reinforcements to his new southern colonies. I get no luck and my free Industrial tech is Steam Power, not Nationalism. Riflemen are about 12 turns away and I cannot trade for them. Frankly, I've neglected Xerxes a little as with my custom domination rules (30% territory and population) I am focussed on curtailing Japan from a domination victory on the other continent and France still hates me and I need to avoid culture flipping my French conquests. Xerxes uses his successful doomstack in conquered Byzantine (reinforced by naval transports), declares war on me and causes sufficient mayhem with his pincer attack into my north and south flanks that I quit the game with riflemen, and hope, both well out of sight. It is not my finest hour but it has been a fun game to again test MPPs replacing MAs and to watch Xerxes do Xerxes things.
Summary: Xerxes turns into his usual Deep Blue-esque, five-headed AI god mode as he approaches the Industrial era and goes from pretty much 7th ranked Civ out of 10 to being well on course for a domination victory following his perfect timing to Cavalry rush the Byzantines and myself. I've played a lot of Civ 3 in recent years and I never see any other AI Civ go from middle of the pack (or in this case, bottom half of the table) to threatening for the win with anything like the consistency Xerxes manages. The other AIs (excepting maybe China) lose their heads in the Industrial era, go for stupid wars of attrition and let the human player catch up and overtake. Whereas Xerxes actually consistently close the gap to the other Civs (human and AI) in the late game from a weaker powerbase and, can often leapfrog these rival Civs if circumstances can permit it. It was almost a pleasure to be spanked by his pincer movement in this game. It didn't feel like losing to the goofy Civ3 AI and I can only applaud my AI overlord.
Anyway, this is a longwinded way of saying if you want a fun late game AI challenge in your customised games then replicating the Xerxes's traits and 'build often' attributes (as laid out by Quintillus) is definitely worth a try.