There can be residual coordination issues as a result of EPM, and it may be that performing with increased speed causes coordination problems for your horse. Has this problem come on suddenly, or has it developed over time? Was he in regular work since the successful treatment of the EPM? If you treated him 10 years ago, how old is your horse now? If he is in his late teens, then arthritis in the joints could be a contributing factor. It would be a good idea to have a veterinarian examine him and watch him move at all gaits and at varying speeds of each. You should not push him further than he can physically handle. That would be unsafe for you and your horse.