So, here are a few probable offenders:
- Your brightness value isn’t locked, so your lights ‘flickering’ is really just a tonemapping problem. Make sure your brightness is set and locked to a specified value.
- If using MRQ (if not, you should), systems in unreal engine often depend on temporal history to accumulate to work correctly, so make sure to enable some amount of temporal accumulation in the cue before it starts (there is a specialized feature to do so, but I don’t remember what it is called).
- Make sure to crank all the values in your PPV for lumen (especially the max trace distance and lumen scene view distance) to make sure you always have indirect lighting coverage).
From the look of your aliasing, it appears you are doing this live in-engine. I would heavily advise use of the movie render queue to allow for maximal quality, as it lets you increase the spacial and temporal samples and clean up a good deal of that noise.