Okay I am 95% sure that it has something to do with from what direction the sun casts light onto the environment.
In this video you can see, that the parts which are directly exposed to the sun start to shimmer and you can see the black dots/artifacts which are causing this shimmering effect. It is very subtle in this example but I am certain that this may be the issue.
Please watch in fullscreen and full resolution to be able to see it. I point at the effects with my mouse.
As you can see, as soon as I turn away from the sun, the shimmering goes away as well.