I see something interesting the webcast of the NROL-76 launch. At around
T+00:00:55 it looks like flames from the exhaust start jumping around the bases of the ring of eight nozles in a circular sweep. Until this point the exhaust is only below the nozzles, after this point it seems everywhere. The process takes place between 4 and 6 km of altitude, at a speed 250 to 300 m/sec.
The video is keyed to start ten seconds earlier, where it looks to me like the exhaust on the left side of the image starts getting "jittery" before it begins to jump around the bases of the nozzles where they connect to the combustion chambers.
The image samples are for convenience only. If you can, view the video itself. If you need more screen shots, explain what you need and I"ll try to add it for you. Also, remember that YouTube allows you to play at half and quarter speed, and in HD.
What is happening here? What causes this change in the shape and location of the engine plume?
In the GIF the images are spaced by about 1 second, so it is approximately normal speed (though under-sampled)