Wow! My story is much too long to type all of here (I'll try the condensed version) but I had a horse who acted just like that and after test upon test, going to Cornell, consulting with New Bolton, spinal taps, and the whole kit and kaboodle we found out it was Lyme Disease, which, if you know anything about it, is known as "the great imitator". When it first happened he would just go lame all of a sudden. One minute fine, the next it looked as though he was walking on eggshells up front and he walked very stiff legged, like he would throw his limbs out and around. The weird thing was that the lameness seemed to shift around and not stick to one limb so that was the first clue something was off. The next time he went lame suddenly he carried his head very low to the ground and used he whiskers to sense where the ground was and that was when we started thinking EPM. He would walk all stiff with his head down and when he was in his stall he would lean on the wall and keep his head cocked to the side. We took his for a spinal because of the symptoms and he actually laid down the whole way to the vet and then fell off the trailer when we got there because he couldn't stay upright. He failed the neurological exam by the vet but his spinal came back no EPM. The really weird part...2 days later he was normal again for about 2 months. Then it happened again but this time he passed the neuro test even though he was swaying and lame again. They x-rayed him all over and looked for Wobbler's, OCD lesions, and some other things but still nothing. Finally, someone took a blood test for Lyme, and, lo and behold, there it was. We ran him under A LOT of Doxycycline for I think, but it was awhile ago, two months. Twice a day, lots of crushed pills and molasses and gradually he got completely better. He is now ridden daily and jumped frequently, and doesn't have any lingering effects. Just wanted to let you know because lyme disease isn't something people often think about but it is very prevalent in the North East and if it isn't caught in time it can have lasting effects on the joints and begin attacking internal organs as well. Looking back when he first started showing signs was before he went lame and he would do small little things that we thought were behavioral at the time such as refusal to pick up a canter, refusing to go in a certain direction, seemed to forget training over night and go back to square one (and this was a horse who was competing and trained not a newbie under saddle). Does any of this sound familiar for you situation? Take care and best of luck.