LumenRT - flicker

When exporting movies from large scenes, I'm getting severe atmospheric flickering in the distance. It appears to be a fluctuation in the "Clear/ Hazy" setting, but even appears if I turn Hazy right down to Clear and Aerial down to 0%.

I've checked on the usual suspects;

Please let me know what else I should try to fix this.

I'll upload the DGN and LRT now.