I don’t have an answer for you, I thought I did as I think I had the same issue when first loading but no longer an issue. Not really sure if I changed anything or if it was just a shader compilation issue.
Anyways, a screen shot of what you are experiencing would be essential to help. You can get a screen clipping using WindowsKey+Shift+S, then just Ctrl+V in the reply on a new line and it will upload.
I would guess it is a ray tracing noise issue. Does it get better or resolve if you turn down from your epic settings? Assuming drivers are all up to date etc etc.