It sounds to me like you have bracketing turned on with Auto ISO neutralizing the bracketing up to the limit. Here are some ideas to isolate some possible issues.
Make sure your target does not have extremes - no lamps, sun, bright reflections etc that could be excluded as you zoom.
Make sure Auto ISO is turned off.
Don't use flash.
Try some test images with Manual exposure mode and a specific ISO setting - ISO 200 for example with Auto ISO off. Make sure the camera is responding with 10 consecutive images at the same focal length all having the same exposure. Then try 10 images at 70mm and see if they are all wrong.
Take a look at your current images in a Nikon program that displays all the shooting data. Try View NX2 which is free - and make sure you have the latest update.