In looking at 0404 and 0417 one issue may be with shutter speed and the movement of the drummer. If you look at his waist area and pants I think you will see that they appear to be better in focus than the upper body and head which would be moving more than his waist. The slow shutter speeds simply make it harder to determine if it is the cameras AF system. Especially if it is happening with 2 bodies.
The shutter speeds also seem a little low compared to the focal length. One of the things that became somewhat apparent with the introduction of the D7000 is that in order to get crisp images consistently you really need to be at 1.5 to 2 times the focal length with the shutter speed with the increased number of pixels unless your technique and hand holding is flawless. I am sure that is even more important with the even larger number of pixels on the D7100.
Looking at 1009 I am not sure what happened on that one as it is clear that the area in focus is the table behind the guy kneeling.
I think I would head outdoors with some good light and try doing some tests under those conditions using focus priority in AF-C single point and see what you get. If you still are having trouble you may have a problem at which point I think I would do a little more formal testing using a tripod, etc., in order to be sure of my results before contacting Nikon. Although, with it happening on both bodies I find that very unusual based on the track record of the D7100 so far, but anything is possible.
Marc There are always two people in every picture: the photographer and the viewer.-Ansel Adams