I'd go with Mrs. D suggestions.
 

They will be more likely to raise your D and B12 levels and keep them there consistently. (B12 injections tend to be "spent" in about 48-72 hours.)

I'd also like to recommend (once again) the Liza Jane spreadsheets, which were developed to list and track almost any kind of test for neurologic symptoms:

www.LizaJane.org

It does sound like you need a new, more extensive work-up, and someone to look at it with fresh eyes.

The typical injection dose is 1000mcg.

I'd do that, and not the 500mcg you stated above.

If you are doing injections it is probably cyanocobalamin?

That is a synthetic version, that has been replaced by Methylcobalamin(has no cyanide). Oral daily is far more natural and effective than injections. Some people cannot tolerate the cyanide portion of cyano form and others cannot convert it properly.

Frequent injections of cyano raise this specter of potential of intolerance to cyanide.

Vit. B6 makes me tingle if I take a normal amount. I have to take less. Perhaps your multi has B6 or you take extra. This is pretty common. Just another idea for you.