Ex-Wife Unstable, Wanting to Move 300 Miles Away with Kids

My question involves a child custody case from the State of: Oregon

My ex-wife just called to tell me that she is moving herself and my children (who are 15 and 9) 300 miles away. I am wanting to know if she can legally do this?

According to our divorce papers we have joint custody. The only thing that specifies where they live is the parenting time listed which states they are with her during school, which is the arrangement we have had for the past 7 years. Our papers do state that if either of us are to move further than 60 miles from the other we are to notify each other and the courts. I personally moved 90 miles from her 4 years ago, but I did notify her at the time and the courts and I made sure she never had to drive more than 60 miles and none of her time was compromised by this.

Further complicating this is that she has been very unstable the past few years. She has been going through another divorce and at the start of this past school year she moved 3 times in the time span of 2 months, making the kids change school each time, and not making sure they got to school. I spoke with her over and over about how unstable she was during these times and asked her to let the girls come live with me during these times and each time she told me she would settle down. Finally in April she called me up and told me our 15 yr old daughter had been skipping school and hanging around the wrong crowd and she wanted the girls to come live with me.
She gave me 2 hours notice. I immediately enrolled them in school the following Monday and they lived with me until school was out last month.

Now, she just called me today telling me she was planning on moving 300 miles away! She has a history of mental illness and is obviously making decisions on a whim again and dragging my kids with her. I don't know what to do now to stop her from taking them 300 miles away!
I cannot afford an attorney, I saw one earlier in the year and they demanded a $5000 retainer which I cannot afford!

If I demand mediation and we can't agree in mediation will the court look at the situation? With her history of instability I cannot imagine they would let her take them away like this. Does anyone have any advice? I am desperate!