FL- Definately listen to Yoretrapper. I just finished reading through this thread, and everything you posted sounds like (to me) like Mareks.
Marek's is a form of herpes, which can present itself in three forms. #3 is the most virulent, and the most deadly. It creates the legions on the brain, which cause the "drunkenness" you describe. It also has auto-immune symptoms, which make the sick bird more susceptable to other diseases: so a chicken dying of mareks may often be KILLED by something stupid and treatable like cocci.
A good friend of mine just went through this, and lost 2/3 of her flock. She was treating for anything and everything, but the birds just did not get better. Once the legions are on the brain, they are there to stay. When she finally could afford the vet--4 months later--, he was ASTOUNDED the chickens had lived that long. None of the still living birds could walk, stand, sit, preen, or make "normal" noises. Some could still eat and drink on their own. Some honked like geese-probably the result of damaged tracheas when the nerves to the neck muscles were damamged.
Once a chicken exposed to Marek's, it is always a carrier. The herpes is transmitted via the shed dander. Vaccinating your entire flock---at this point----is a waste. Your chickens will either live, die, or get one of the 2 lesser forms (legions in the throat, or legions in the eye--gray/inconsistant iris like you mentioned). The best you can do is make SURE to vaccinate & quarantine any new birds coming in. Otherwise you are on complete flockdown until all your current birds have passed.
Mareks can also live in the soil for up to (I read recently) 65 weeks. So keep that in mind.
SO, SO VERY SORRY for your troubles
