We had two old horses at the barn I used to work at, (horses were there a few years apart), and they both at one point started becoming incredibly dangerous, lashing out, taking off under saddle, the one I never met actually broke my trainers back by taking off, and throwing her into a rock. She was around 60 at the time of accident, so too old to getting hurt like that. His right hind leg I believe would also unexplainably swell up, always in sync with when he would have his worst episodes. And the horse I actually met didn't become quite as aggressive as the other guy, but did get pretty bad, and again his right hind leg would swell up. Well when the first horse was put down, it was discovered that he had a brain tumor, and the trainer believes that the current horse displaying the same symptoms also has a tumor, but won't put him down until he actually does become too dangerous for her to handle, since she's the only one who does anything with him. She feeds all the horses, she blankets, turns out, rides ect. I don't know if he's had to be put down yet, haven't talked to the trainer in a while, but I know with those two there was definitely a change in attitude, not just a worsening of already bad behavior. To me it sounds like the op's horse already had some bad behavior, and has learned that it gets results, so she's gotten worse and worse, and the cycle needs to be broken somehow, but with a person who KNOWS what they are doing, and can do it in the safest manner possible. I would be very cautious about her being in a stall, because as others have posted, its not unheard of for horses to pin someone in a corner, and I would hate for that to happen to you.