Oh wow Sophie. I did see a little of this when we met you at the park. But it sounds like it's gotten worse. I do remember when a dog came to you for a treat and she went at them. It didn't seem like it was out of control or anything. And she seemed to stop when you stepped in.
I'm sorry. I do not have any suggestions for you. I assume you've taken her to obedience training, since you are already working on agility. Maybe she needs more obedience training. Maybe we could take our dogs together to that, because I KNOW Zoee needs another class.