"Hey, I'm glad you dig your ex. But I'll tell you, you asked me not to talk about mine, then you talk about yours constantly. And I want to hear about yours about as much as you want to hear about mine. So figure out what you want to do. You can talk about yours and I'll talk about mine, and we'll have a good time trading ex war stories. Or you can not talk about yours and I won't talk about mine. Let me know which is better."
Then just remind her the next 4-5 times she does it. Then if she continues to do it after that, make an obvious in-her-face ex reference when she does it:
HER: blah blah my ex was SO annoying!
YOU: I know. My ex was great but she did this annoying thing with her teeth. I have no idea why she did that.
HER: OMG I told you not to talk about your exes around me!
YOU: I thought we agreed. If you talk about it so will I. You keep talking about it, so I figured it was fair game. I'm just following your lead here.
As for the instability... well. Make sure you've read this article:
Spotting (and Avoiding) Cluster B Women