How do I stop headshaking during dressage?

After a course of Equiwinner has cleared up the headshaking generally, the horse will sometimes headshake just during dressage exercises. The cause of headshaking has been removed but the horse has learned to headshake along with the dressage routine. The horse has to be taken away from the menage and given different work, such as a few long hacks. This should break the learned headshake response when the horse is returned to dressage.