Quote Originally Posted by Orda Khan
In one battle I sent my newly come of age family member with only his bodyguard to meet up with one of my field armies. He was instantly attacked by a Gothic army of 800+. I targetted a HA unit and coralled it to the map corner where it was killed/routed. Next I began to lead the infantry (a mixture of runaway slave spears and Gothic spears) around the map, trying to encourage their general away from safety. Sure enough, after a while he attacked and I defeated him before his army arrived. When they did it was only a matter of some flank attacks before they were routed from the field!
I gave up playing after that.
I've experienced things like that. They can be quite fun but not really a reason to stop playing. What you are doing is basically an exploit and I would not use it to fault the AI. It's programmed to fight a pitched battle, not a chase a lone cavalry unit (and having chased lone horse archer units, I know that such chases are not trivial). Once you've killed it's cavalry, it's not really a "battle". Given your manouvrability edge, he's brought a knife (ok, many knives) to your gunfight.

Rather than a problem with the AI, I think it points to problems with the settings: cavalry is too strong (mod your bodyguards' hit points down from 2 to 1); spears are too weak against cavalry; and morale is too low. Try those tactics with a "realism" mod like EB or RTR. They might still work - and in reality a body of what are effectively knights often could beat up large numbers of mediocre infantry. But I would not count on it and you certainly wouldn't have many cavalry left.