Your anti-aliasing is too low, it will cause a lot of noise and flicker. Try at least 4AA, but you may need 6. Start with default values for Pixel Noise Threshold. Also I suggest using Robust Adaptive Sampling if you can, and upgrade to 4.4 if available.
If you still have flicker after increasing AA, you may need to increase Cloud GI Quality (in GI settings, GI In Clouds), and possibly Ray Marching Quality (in cloud layer's Quality tab).
- Oshyan