I agree, it sounds like it could be some type of poison or even Botulism.
Marek's disease also comes to mind.
Check feed to make sure nothing is bad or moldy. 
Corid is a Coccidiostat, it's not an antibiotic.  If you are treating with Corid (Amprolium), do not give Extra vitamins/supplements that contain B1(Thiamine) since there can be a contraindication.
If you want to treat Neurological symptoms, use Vitamin therapy giving 400IU Vitamin E and 1/4 tablet B-Complex.
Hopefully you can send the body or tissue for more testing to get more information.