I would a million times check her for ulcers or a cyst - either of them - how long have the symptoms been going on? What does her diet consist of - if you can list the ingredients in any grain? Whats her daily life set up as - has anything big changed in the past few months? Ulcers are usually caused by high exercise levels or high stress levels, so frequent trailering or hard work - but it can also be caused by an improper diet or some horses just get them real easy.
I got her last October. Where she came from she was on lush pasture the majority of the time (I'm talking LUSH pasture). She was not on grain at that time. I think they would occasionally take the horses in so she was used to being in a stall. She was not worked consistently with the lady I bought her from (she bucked her off once and she got scared, plus she had too many horses and too little time). First time I saw her she crow hopped twice with the old owner on (I got on her and she didn't buck for me). This person only had her for around 3 or 4 months....and my mare was at the breeders her whole life until then so I am third owner, but I have owned her longer than her second owner now. To my understanding, the mare had three months training on her when she was with the breeders and did excellent in training. The breeders got her back and started to work with her and they said "she was just too much horse for us to handle." I don't know exactly what that means and I'm not sure what kind of feeding schedule they had her on there.
When I got her, my trainer and I worked with her every day, except one or two days off for her. For the first three months as I stated (while I was using a saddle that fit her) she did not buck once.
She eats every day on the dot at 4am and goes out to turnout at 5am. They bring the horses in at 4pm and they eat again at 5pm. At first we were only giving her a handful of grain 2x per day but it has been adjusted to fit the amount of work she has. My trainer did mention that if I was at my own facility she would recommend putting my mare out on grass and see how she behaves then. Where I am boarding, in the summer the mares do not get to go out on grass pasture - geldings only. I was coming out at night and letting her out in the pasture for about 30minutes so that she could get some grass (she ate like a mad woman). As far as what she is being fed I would have to double check as I embarrassingly forgot the name of the grain my mare is on. I will post later with the answer to that. I did have her selenium and vit E levels tested and she was at a perfect level (the chiropractor told me to put her on supplements but I was not going to do that without testing first and good thing I did).
The only big thing that has happened is that her best buddy got in an accident and died. A very sad story. My trainer was working with him and his owner. They loosely put side reins on him and walked him a few steps. He was fine and then out of no where he went up and over and landed on his back. They thought he was dead until my horse whinnied and he got up, staggering around looking for her (all the horses were in the barn, and our indoor arena is attached to our stalls and isle ways, so she witnessed the whole thing). He did this numerous more times, getting up every time she called to him. Took the vet 45 minutes to get out there and they had decided to put him down as he had bleeding in his brain etc. It was a few weeks after this that she started biting at her sides. She also was not only turned out with him, but she is on the end stall and he was her only neighbor for the whole year I have had her. She tried to pull me into his stall many times and when I let her loose in the arena she goes to the back stall in the arena where he was put down.
As far as exercise levels she is worked consistently. She has one day completely off and her other "day off" is just a very casual lunging day. She's really not being worked hard because her bucking issue is so severe.