Horse wont stand still?

I have a 6 yro thoroughbred off the track and when i first got her i had to get her used to being in cross ties. It worked fine and she always stood perfectly still in the cross ties while i brushed her and tacked her up etc. Now about 4 months later the weather is getting nicer and she will not stand still! when i have her in the cross ties the big door is open and she can see out into the field where the other horses are and she wants to be out there. I dont want to have to close the door all the time but she just dances around in the cross ties constantly. I tried lunging her and getting her energy out but that didnt help either, any ideas? She also does it more when i am brushing her then anything else.. she does it no matter what way she is facing as well.