It's very possible that the shutter is sticking. I had this problem a lot of years ago with a Yashica 35mm film camera and lost a lot of shots to the shutter misbehaving, an expensive proposition with film. Anyways, the trigger seemed to have been a trip to a hot and fairly dusty location in Aruba, maybe the result of some sand or something getting caught in the mechanism, though that's speculation. Net effect, this is a mechanical part of the camera and can fail.

My initial reaction to your post was flash sync speed, but your first example as a 1/100 shutter speed and that's well below a flash sync problem with even the worse remote radio triggers and I doubt you're using them (yet). That rules the sync speed out, I think. The card suggestion above also doesn't make sense. It might, maybe, be the reason for one or two images, but that seems unlikely at best and not believable, at all, for any volume.

So, long story, short, I think you need to send it into Nikon. Hopefully it's a warranty repair.