Hello: I'm leaving shortly for a photo trip and I cannot get 8fps on my D700. I've checked the settings and everything seems fine. I have the MB-D10 with fully charged N-MH batteries, camera set to use MB-D10 first, RAW, Ch mode, Single point AF, etc. But, I still cannot get 8fps.
Consider getting an EN-EL4a for the grip and you will never have that problem again. I know they are quite expensive and also require a separate battery door and a special charger but I feel the cost is well worth it if one can afford it.
EN-EL4a or 8 AAs are the answer to achieving 8 fps. EN-EL3e batteries max out at 6 fps, I believe. The battery holder for the fist full of AAs comes with the MB-D10. You'll need a special door to be able to use the EN-EL4a in your MB-D10. Also, Nikon recommends not using some AAs -- because they lose power at lower temperatures and thus will not provide the faster rates for long, if at all. As Jim says, the EN-EL4a batteries are not cheap and they require a specific charger. As an old employer of mine used to say: "The answer is 'Yes,' but there's a cost associated with that!"