Steps to reproduce:

1. Open attached project "Skybox_new.zip"
2. Open scene "test"
3. Build project
4. Run it on highest allowed resolution and ultra settings
5. Notice that sometimes top of skybox flickers

Note: repro doesn't seem to be consistent, sometimes multiple flickers happen right at the start, other times after a minute or two. It's also possible to repro this in editor with Metal Editor enabled.

Doesn't repro with DX11 or OpenGL

Reproduced with: 5.6.4p4, 2017.1.2p4, 2017.2.1p2, 2017.3.0p2, 2018.1.0b2