I've had a similar problem where it was when a buggy army moved into sight it crashed... But apart from that, its difficult to say.

Try using the cheat to get rid of the fow and scroll over the AI cities, see if that makes it crash.

Something else you can do is after backing up try elliminating the possible sources of the problem one at a time. That way, when the bug dissapears, you'll have some idea as to where it is.

And welcome to the .org jjca