I am now wondering if it is a kind of LOD issue. It is hard to see the shimmering itself on this low quality headset video capture, but when the buildings over my right wing are bright white (reflecting sun) then they are shimmering significantly in the headset. As I slew forwards and backwards, the bright reflections (and shimmering) appear beyond a certain LOD ring - not instantly, but fades in approx 1 second after I move (as with other LOD effects).
When noticing this in sim previously, I had assumed it was a reflection angle issue and therefore as I moved forwards the sun reflection angle on the buildings was changing, but this does not appear to be the case - slewing vertically up and down to change the reflection angle has no impact on the shimmering, only lateral LOD distance does.
Is there any way to use the dev mode to see what LODs are changing at what time? then I could be more accurate in describing exactly what is triggering the shimmering.
Edit - a different and zoomed in clip of the same effect somewhere else: