What is going on with my Nikon D7100 and Sigma 150-500mm lens? Major focusing issue?

I've had my Nikon D7100 since late last November. I used to shoot with the Nikkor 70-300mm VR and the shots came out extremely sharp. Back in January I upgraded the lens to a Sigma 150-500mm. There seems to be nothing wrong with the lens, I've tested it on my old Nikon D5100 and the shots are fine and sharp. But it's a totally different story with my D7100. I've fine tuned the auto focus and tried many different settings but absolutely no shot, even in gorgeous lighting at point blank range, is even close to being focused properly. I know what I'm doing, I've been into nature photography for a few years and have experience with DSLR's even longer. But this is seriously frustrating that I have equipment that it completely capable of fine shots and they aren't cooperating like they should. Strangely, today out of nowhere the camera started acting bizarre and would take black shots when it wasn't supposed to, shots would come out too bright even in the correct settings, and the buttons and focus would lock up often. It had not done that before. So I'm wondering what I should do. It's really odd to me that the camera focuses fine with any other lens and the lens focuses fine with any other camera. I don't wanna sell the lens, I don't see a point. But I feel I should do something about the camera since it's really acting up. Should I contact Nikon? It's like the camera is totally rejecting the lens. So any suggestions on what I should do would be great.

The fact is that it's Sigma and you're using it on a Nikon, it's called "compatibility issues". You can try returning the lens back to the store (without ever mentioning about it working with the D5100) or simply limit its use with the D5100 (and forget using it with the D7100 ever).

You might have the copy my friend returned. His shots were shocking. Fortunately they exchanged it for a Canon 70-200f4L.