I recently bought an MB-D10 for my D300s. I had a very similar situation to yours, although with the added feature of the camera power cycling every few minutes. After much frustration I found that it was a battery issue. I have a couple of third-party EN-EL3e batteries, as well as Nikon. I found that the output of one of the batteries was 1800mAh, whereas the Nikon batteries are 1500mAh. Once I changed back to a 1500mAh battery both in the camera and the grip, it all worked.
Don't know if this helps you, but with much embarrassment I will put it out there.