Have you had genetic testing for inherited PN?
http://neurotalk.psychcentral.com/thread121564.html

Are you taking any supplements? Do you know your B12 reading and Vit D3?

Slow decline of mitochondria, may not show up in muscle biopsy.
As I understand mito damage, the sample must be done a certain way, etc. So it is possible you have some mild mitochondria damage that is ongoing.

I would look to diet and do gluten free for about a month, and see if your start to notice any improvements there. Gluten intolerance is sneaky, and difficult to pin point with blood testing sometimes. I have found for myself that certain foods can start a burning attack that is very intense. And when I change that, the burning goes away. Nightshade veggies do it for me and some spices, and some tomato products.

I know it is frustrating.. we all experience that feeling.