There seems to be an issue with unit movement when the game is slowed down to "Slower" speed in single player. This is most noticeable for Armies, but even single units suffer from the problem. In armies the army tends to "dance" around rather than moving along toward it's destination. In the worst cases the only way to get the army moving is to speed the game up to normal mode again. When the game speed is set back to "Slower" again, sometimes the army will then continue moving (slowly or course), other times it will go back to dancing.
Perhaps the method used to slow down units relative to the game speed is breaking the pathing logic?
Cheers, DaBang.