4th D600, same problem.

TOF guy wrote:

You have limited course of actions:

• Exchange the camera. Main caveat: the next one is likely to show the same problem.
• Return the D600, wait for a few months that Nikon solves this Q/C issue and get a new one. Caveat: it may take a while for Nikon to find the solution, if ever.
• Send it to Nikon for cleaning. Caveats: you pay for shipping in the US to the service center (return is at Nikon's expense). The oil may come back and additional cleanings may be at your charge.
• Find a local photography shop which will clean it up (at your expense).
• Clean it yourself. Caveat: you scratch the sensor, you're on your own to repair the sensor. You'd be unlucky if it happened to you but you wouldn't be the first one. Note that Nikon recommends against users doing any cleaning of the sensor themselves (does not mean you have to follow their advice).
• Give up on the D600
  

Sorry to read the issues you have. You have every right to be upset at Nikon's poor handling of QC.

--
Thierry