Thank you for all the suggestions guys and sorry for not detailing the problem correctly!
This is running with a dual Xeon 2699V3 with 32Gb of RAM and a GTX 1070Ti, so I don’t think it is a performance issue. At first, my suspicions were the GPU driver, I updated it and it didn’t solve the problem. I’m also running it remotely with Parsec. Can’t remember if I actually tried locally but I guess I already tried that just to rule out Parsec and if I remember correctly it did exactly the same thing.
The artifacts you see on the bottom of the first frame are always present when moving around the viewport and sometimes shift to other corners. What you see on the second frame are the flashes that occur every 3 seconds or so.
I’ll keep an eye on ram usage and get back with a video. Don’t think the problem lies there either but I’ll check further.
EDIT: Ram usage is at 23%. No problems there apparently.
EDIT 2: The scene is the Archviz scene converted to Lumen and UE5.
EDIT 3: Video here: 2021-07-17 20-41-20