Could stil be an ALV or even a deficiency, I suppose. When was your property built? Heavy metal toxicity causes ataxia and paresis. It can be a lengthy progression in it's symptoms depending on how it's ingested e.g. if a bird ingests lead shot it's going to manifest symptoms more quickly than if it's picking up tiny particles in the soil over time. A blood test can determine if it's a metal toxicity.
Botulism or algae intoxication could be another possibility although these tend to kill pretty swiftly.
Sorry it's taken me a while to reply. I had to get a paper done for my course and as usual, left it to the last minute