My D7000 is not working properly. No matter what lens I have attached to the camera body, when shooting in manual mode, the shutter speed changes itself after every shot. And not just a little, it makes drastic changes. Why is that? Is there a setting to turn that off? The shutter speed should remain unchanged unless I change it. The aperture stays consistent in manual mode, but not the shutter speed. Please tell me how to fix this. Perhaps I have a defective unit. In fact, shutter priority and aperture priority are also having issues. I will take one shot that comes out fine, not change anything with conditions staying the same, and the next shot is way over exposed. Please help. This is getting frustrating.