A poison or toxin is possible. I agree with trying to supplement vitamins to see if you get improvement. Epilepsy does happen in chickens. If that's going on then sometimes it can be triggered by stress (encompasses a lot), surprise, flashing light or light changing from bright to shadow.
Here is a thread on another site, similar, seems like medication helped control it but then the bird wouldn't eat.
http://forum.backyardpoultry.com/viewtopic.php?f=5&t=8014143
Here is a thread on another site, similar, seems like medication helped control it but then the bird wouldn't eat.
http://forum.backyardpoultry.com/viewtopic.php?f=5&t=8014143