Many thanks to all of you for all your replies/suggestions. If the next discussions/tests with the vet bring anything useful, I'll let you know.
In reply to hoseforeeve1... No it's not only when I pick his feet, some times it happens just like that! Thanks again!
My horse, a 10 years old very laid back Andalusian, occasionally falls down when tied up. I get no warnings, his head doesn't drop, his legs dont seem to go wobbly, one minute he is up the next he is down. He recently did it when i picked his front feet, twice within minutes... I have watched...