This reminds me of a mare that had this problem. She was fine for years and
then started to just fall flat. At one point my wife was thinking of riding her
when she went down at a full gallop with me on her about 20 feet from my wife.
She would trip onto her knees and grind her face in the dirt even at a walk or
trot.
Got to the point where the owners were going to retire her at a young 11 years
old. Working on a hunch I ask the ferrier to shorten her toes (or lengthen her
heels depending on how you look at it). This helped a lot, I followed up with
four months of conditioning. She turned into a really nice trail horse.
I'm not saying the same recipe will work for you or that it's not a medical
problem. What I think I'm saying is that the horse may benefit from a rider who
can tell which foot is where at any given time and trace the fault.
Good luck on this one.
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E-Mall BBS Poway California 619 679 1158 & 619 679 9327