I noticed something similar, getting weak between meals. I also get an increase in my symptoms if I eat a big meal. The solution that works best for me is to eat a handful of nuts, seeds and a fruit between meals if I feel weak. I think my weakness is nutritional related since not all foods make this weakness better.

I have also started taking the B-12 recommended by Annie, the Jarrow brand with Methyl B-12 and I am seeing a difference in the way I feel. I have fewer afternoons with weakness. I can feel the difference between this B-12 and the other brands I have tried. It has also extended my daytime awake time by approximately 3 hrs! I am wondering what the safe limits for this vitamin is. I started out taking 1 tablet (1000 mcg) every 3 days, then increased it to one every day. I have taken two a day on days when I feel tired and found that the B-12 makes me feel better within an hour or two.

(Thanks Annie!)