Is she a shorter backed horse? A lot of times it's harder to find a saddle that fits you and the horse with this issue, and natural hindquarter movement can push the saddle forward in this case. Verona's post could be very helpful for this.
She isn't too terribly short backed. She's just flat backed. Hardly any rock to her.
I'm going to get a crupper, just because my saddle slips enough to justify if, whether or not it helps the girth issue. Got myself some Body Glide at Walmart today. Will experiment with girth tightness.
I know you have a specialized saddle, I don't know how long your girth is. I had an issue with some sores when I was using a mohair cinch that had leather backers. It turns out that the leather backers actually rubbed some and that is why I only noticed it on really long and hot rides.
I bought a cinch that was longer and thus moved the backers higher on my horse and I have not had any more issues with the rubbing.