Any possibility that he ate locoweed in his past before you got him. Seeing it is mustang that was running wild and food got scarce, ,that maybe be the case. This is a classic reaction.That poison forms nodules along an artery that runs along their chest. At times those nodules break loose and then travel to the brain. Mostly that happen when they get worked up sweaty or excited.
There isn't a clue given when it happens, but there is no cure either. Typically there is no communicating with the horse.
Might not be the problem, but a possibility.
I know what you mean about pride and loving the horse. I bred and raised a nice haflinger mule. I had it 12 years and just had this nagging thought that some day she would hurt me bad. I had a runaway and I had to bale off because we were at a dead run entering into a thick grove of aspen that I couldn't possibly survive. I ended up in the ER with a few broken ribs, concussion, and bruised internal organs,spleen,kidneys and such.. I was 70 at the time and didn't heal quick. She is on a pack string now and no one rides her.
I had sold a guy a good thorobred mule. Gentle as could be. He turned it out on 80 acres infested with loco weed. In 6 months he called me and told me the mule tried to kill him every time he went in to the stall. He wanted me to buy it back. I went down, got the mule out, saddled it, and led it out a ways from the barn to a soft area and climbed on.It just stood there. I clucked to it and touched it with my heels and it blew up.
On the third jump,I shucked my stirrups and kept going up in the air as it went down. I came down and looked over .It's feet were 4 ft off the ground.It was squealing, bellowing, braying, going crazy. It did that a minute or so, then froze up. Stood there and then went off running out into the pasture and repeated the whole thing again.
I waited a bit and then went out and caught it up. I was able to lead it back in. On the way, I saw all the loco weed. I picked a bunch,and took it back in. I told the guy to take it to the Ag Extension Office and have them tell him what it was.Then take it to a vet and ask them what it did to a horse or mule.I told him if that wasn't the cause, I would buy the mule back. I never heard from him again,but heard the mule went to an auction.