EPM can look like, and share neurological symptoms of many other diseases. Any muscle or organ from the lips to the tail can be involved, as can attitude and behavior changes. This is why it is so hard to diagnose. If you think you might be dealing with EPM, have the vet pull blood for an IFAT test through UC Davis.
There are two other sources for helpful information besides the ones Cindy gave you: www.EPMhorse.org
basic information on the disease, tests, symptoms, drugs, and rehab - a lot of reading, but well worth the time. It also has a blog about the rehab of an EPM horse. EPM : Equine Protozoal Myeloencephalitis.
Read before the next time the vet comes out, so that you know what questions to ask the vet.