I've done some number of very long exposures and two of my friends who are into long exposure photographs have done many more. We all own D800 (not E, but I would not expect any difference in this regard). This is what you will get with D800. It is a temperature noise - the longer sensor is working the hotter it gets and it produces such noise. It is not that bad on less megapixel cameras.
At this time, based on all the reading I've done on the subject, I am forced to use Long Exposure NR which doubles the exposure time. This is unfortunate as it leads to fewer opportunities to take a shot you want. Also, I think the only way out of this (if you realize that your Long Exposure won't exposure the way you want) is to switch off the camera (and maybe even take the battery out) and then switch it back on.
If you do not use Long Exposure NR then you will have to deal with this noise in post production which will take a while since the noise produces spots of different color and size.