Well, as it has happened like 7 times in a row now, I KNOW its when client rulers die. Nothing except FM's banishes it (no armies, unfortunately), and even then the problem isn't really banished, cos if the FM leaves again, it'll return. But I was also interested to know, whether dead client rulers aren't supposed to be replaced? None of mine have ever been, but since it works so smoothly the first time you get one, I keep wondering why new ones don't replace the deceased...