Hey cat,
There are a LOT of really really smart people here, who will be able to point you in the right direction.
I'm sure they will be around shortly.

I'm in the same little boat as you.
I've taken weekly shots (1000mgs) of b12 for a year, that's how long it took my levels to go from 220 to 900.
and out of the blue
--the only change was on my good days I decided to get back into an excersice routine--
........and out of the blue they plummeted to 460.
with no explanation.

start looking into
subacute combined systems disease a.k.a. subacute combined degeneraton.
don't give up, you deserve answers.
My doc -on Long Island - was the fourth doc, (2nd neuro) to pick it up.

good luck
lah.