Thanks. Using "functional b12 deficiency" to look around is very useful.
I'm going to try to nail that down to make sure that is what is going on.
Also as far as root causes I'm going to try to get my ND/MD to take a look at my heavy metals situation.. that might be relevant here. In particular I'm seeing that "mercury can block the methylation cycle and thus induce a functional B12-deficiency":
(not the most reputable reference there but it gives a good explanation)