Thanks Arralen and everyone else. Though I'm sad to see a potential nasty bug crop up, I am relieved it wasn't something specific to my box alone. It will hopefully be squashed soon and then I'll have to try and figure out how to handle those thousands of enemy units cropping up on my borders.
I did suspect the large armies might have something to do with it as the CTD only happened after huge 1000+ armies started showing up on my borders. But I lacked
the technical know-how to track it down, so thanks again for all the help.