I am having this exact same problem, I have even reset all settings back to Nikon default (holding ISO/WB, Reset Custom Settings, Reset Shooting Menu). I have checked b4 easy exposure compensation it is OFF. All bracketing is turned off, I seem to get two over exposed frames then two under exposed frames and this repeats. I called Nikon and they had no ideas, I uploaded example photos for their review hope to hear back from them on Monday. I tried to narrow down the problem by shooting in Manual mode, fixed shutter, aperture and ISO and I still get this alternating exposure. I checked the Exif data and all photos have the same settings. My D3s is very well taken care of and has 94,000 shutter clicks, any thoughts as I have run out of ideas.