There was some discussion on the ARABHORSE-L list a while back regarding
horses that did this. You might try sending a note to them about it and
find out what comes back. It was a woman whose gelding was doing the
same thing and there was some concensus that others had had some
experience with this. By now they may have some really good information
about how to handle/cure the situation. I don't remember if he had
actually had a seizure, but I do remember the falling asleep when tied
thing.