We figured out she was girthy in a couple of ways. The first was that she destroyed her blanket one night and the only extra I had while hers was out to be repaired was the belly band one. She was immediately better to ride the very next day. We kept the belly band blanket on her and she got to the point where she would barely buck for a stride or two after the canter transition.
The second point, was that we started her jumping. (She was 4). We always lunged our horses over fences for a few sessions before they had their first under saddle jumping session, and without fail, she would warm up w/t on the lunge, buck a few strides into the first few canters on the lunge, and bucking explosion after jumping even a little crossrail on the lunge. So we started thinking that maybe she wasn't cold backed, but girthy. I had been riding her in a regular leather girth with an elastic end with a fleece girth cover. We switched her to a synthetic girth with no cover. Our thinking was that the girth would "slide" a little back and forth and not feel so confining to her. Again, immediate improvement, but still not perfect. So along the same line of thinking, we did a surgical blade clip along her girthline. After that, she would RARELY even hump up her back for mounting, canter transitions, or jumping efforts. After about a year, she was showing local shows and cleaning up, with never a buck in sight.
Anyway, hope things go well with figuring out what's going on with your girl. It's worth trying some things "out of the box" like we did to see if it helps.
edited to add--the girth that we used for her was the generic synthetic girth from Bartville Harness in PA. It had a bit of a waffle weave texture to it, elastic on one end, straight not contoured, and only cost $35.