Just some ideas that may be worth considering

Is there an additive or something in her diet that makes her behavious worse? Just an idea, but being an educator and a mother, i have seen how kids behaviours change from eating different food.

When she is being an angel, can you guys sit down together and write up what is acceptable and what is not. Also come up with rewards when she follows the rules and consequences when she doesn't
By her participating, she gets to take ownership of the rules and hence her behaviour.

Is there a time in the day when her behaviour is worse? Perhaps keeping a diary of when the flare ups are occurring. You may find a pattern emerges. I know that my DD goes feral the moment her sugar level drops - so I know that as soon as she starts, I give her something snacky to bring it back up.

Working out the triggers and how to prevent the situations occurring in the first place is far better than going into war games with her.

By understanding the behaviour and why it is occurring, the we can best prevent situations occurring in the firstplace. Also by having a game plan in place that she you you have agreed upon will mean that once a flare up occurs - you will have a direction to follow.

HTH