Your horse appears a) to be suffering from NARCOLEPSY - a condition in
humans that makes them fall asleep temporarily. We had a horse in the barn
that used to do that, too, but he never went down. I believe it has
something to do with the interruption of electric impulses to the brain. I
read something about it in a way back issue of Equus which I will try and
look up later. And b) it sounds as if he was having a type of epileptic fit
when he went down. It might all be tied in together.
Definitely, don't ride him if he's going to go down on you, but try and get
a second opinion or ring one of the teaching vet. schools. None of my vet.
books even mention it. Good luck!
Diana Rz.