I'll preface by saying that self-cleaning one's sensor is easy enough and important to know how to do. With that being said--send in your D600 for repair. A total pain (i just sent my own d600 out), but in this case Nikon will likely do more than a cleaning. They have been replacing shutters, including my own. Bite the bullet and hopefully they'll fix it for good. Just pay the extra for shipping insurance and pack it well. My shipping costs with $2000 in insurance came to around 50 dollars.