Long story short: a mom I've met who doesn't eat gluten was telling me her symptoms that led her to figure that out. Note the overlap with PD:

1. she was having "cognitive issues", such that, she went to a neuro
2. she was weak, could not turn on a faucet nor lift a container of OJ or milk out of the fridge with one hand, had to use both
3. this was what got me, as it's a cardinal one-of-the-four classic PD signs: slowness. She said she just couldn not move, wanted to, but things happened verrrry slowly.
4. also had gastro issues but as we were at a teacher breakfast, I didn't ask her to elaborate!

She went to a variety of docs all of whom found nothing. In discussing this with her mom, who is a doctor herself and was currently doing the elimination diet for her gastro issues, the mom suggested trying eliminating foods to see if that helped...first thing my friend cut out: wheat. In one week, she said, she was feeling a lot better, and now the whole family is gluten free and she is feeling normal again. She's just over 50.

We tried going wheat free but simply could not do it. I now realize that the whole family has to do it: it is simply too tempting to have all the wheat products lying around and I'm a baker and the things are just too doggone good! We are going to try to go gluten free again starting today and see if it might help.

The good thing, my friend was telling me, is that every restaurant and grocery store she goes to has entire sections of GF foods, so it's easier to cut out gluten than it used to be.

Rick, I know you've experimented with the wheat thing, does this ring any bells for you?

A different friend who was finally dx'd with Lyme after several years of docs telling her she was nuts...the symptoms she has are also disturbingly similar to PD: she can't follow instructions, there are the cognitive issues again, she has to read them several times before she can do them, she is incredibly tired all the time...also weak...hmmmmmmm.