Have you checked the horse for a magnesium deficiency? To me, your issues sound a lot like this could be the problem.
Not all magnesium supplements are the same. One of the better magnesium supplement companies is:
Their website has a long list of symptoms, which amazed me because this deficiency can cause so many various negative behaviors, which can be corrected with a good magnesium supplement. I tried this product out when one of the mares, who had a girl crush on another mare, became very hostile to the other mare’s foal. The magnesium supplement solved the problem – – the formally aggressive mare still had a girl crush on the mother, but she calmed down and left the foal alone.