D4 colour variation during night timelapse

My D4 sometimes has a nasty colour variation between individual shots during night timelapses. Note: WB is fixed and Active D-Lighting is off.

In the example below you can clearly see a difference in the colour of the sky and clouds (if you make an overlay, colour difference is visible everywhere). Changing from incandescent WB to sunlight or Kelvin WB makes no difference.

The NEF output is fine when viewed in CNX2, but the JPG is not, as you can see in the example below:

It's definitely not a software problem. The in-camera JPGs are really different. That also makes the use of the in-camera timelapse mode pretty useless.

Is anyone (D4/D800/D600) experiencing the same problem?

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Regards, Fred Kamphues www.fredkamphues.com