Well....some never find a cause. But a cause is worth looking for because if you remove it, you may get better at least not progress more.

Have you looked at this:

1) food triggers? including gluten, and the nightshade family of veggies? Doing a journal with what you eat may shed some light on these. jccgf just posted again, her neuropathy/gluten links...
here they are:
http://neurotalk.psychcentral.com/thread115697.html

I myself get burning from potatoes...and it took me a LONG time (years) to make this connection.

2) evaluate all potential mechanical problems with feet and ankles.

3) make sure you have normal blood levels of Vit D and B12. I just put up another thread yesterday on neuropathy pain and Vit D. A small study, but an interesting one. It is on this page.

4) any vaccines this year? Vaccines may start an inflammatory cascade of autoimmune reactions in some people.

I'll say this, my combined arthritis and PN give very confusing signals to me sometimes. I just started grapeseed extract (again), and at 200mg a day find that my burning is better. I typically use grapeseed for allergies, but lately I've been reminded that the RSD board has used it successfully for their nerve pain. So I searched it again (for Bob's tooth issue) and found many papers on helping inflammation from 2009 on PubMed. So I have gone back to it and it is helping with my night pain.
This last month, my feet were very upset by the constant LOW pressures and storms/snow/cold we have been having.

5) Cooling rubs like Biofreeze really help at night and used daily the antioxidant Ilex extract in it may take down some inflammation.

With all the toxins and other nasty things in us, who knows.???

Melody put up this YouTube yesterday, everyone should watch...after watching THAT PN sort of comes into more of a focus as to why idiopathic may be the final answer:

http://www.youtube.com/watch?v=W5n4HhQr25Q