Well, um....

If you are taking B12, 600+ is not sterling. It is okay, but for people having neuro symptoms it would be nice to be a bit higher IMO. B12 has to get into the CNS...and the higher the gradient from the blood serum the better it crosses into the CSF.

I just read that toxicity from Flagyl can cause brain lesions.
Let me see if I can find that again.....

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2963744/
also there are reports of unilateral lesions on Google as well.
It is an example of a drug actually damaging the brain. So who knows
what yours could be from?

I'd take some acetyl carnitine just as insurance if you aren't already. Start at 250mg twice a day.

The D is good... not perfect but very close to my results too.
Considering how many people have results in the single digits, 43 is okay. My doctor wanted me at 70 but I've never made that goal. 50 is the "new" low norm now with the new research.
Given lab error possibilities you are very close.