This is exactly what I began to think was the culprit for me. I am nearly convinced it is the MMT, a gasoline additive with manganese as its base that replaced lead. It has been around for years now. The Afton Corp. manufactures it. The EPA has "suggested" that they provide studies on health effects, but they go on unabated. Canada, I believe, has banned it. The Physician's Committee on Responsible Medicine recommends banning it until public health studies are done.

From researchers at U of California Santa Cruz:

Manganese Exposure May Speed the Appearance of Parkinson's Symptoms...

Sadly, there are many more articles like the one Rick cites linking MMT and air pollution to neurodegenerative diseases.

-Laura