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.
So maybe your cinch is a little short?
I went from an 18in to a 24in.
Proud owner of ~Mana: 6yo Arabian gelding~Pearl 13yo Arabian~Danzer 14yo Arabian mare~ Tiny mini filly