If you have any idea, thanks.
My collegue had the same problem while working on a map for our demo… We ended up reverting to 4.6.1 as it was “not happening” in 4.7.
I experienced something very similar to this with UE4.6.1.
What was happening is that the engine was doing an LOD switch on distant objects, and the lower resolution object had polygons which would be on the same plane as other polygons. In my case, I was drawing textured quads on top of the terrain, and zooming far away would cause the terrain to switch to a lower LOD which has polygons which intersect the textured quad mesh.
The fix was to just increase the distance from my textured quad from the terrain so that a lower LOD wouldn’t cause z-fighting issues.