ok that makes sense. When I put it live I used the All-In-One Migration plugin which copies the database and files which may explain why I still see the flickering on there too.
However, I dont see the flickering on this site or the speedtest site or any of my other GP sites so I really dont understand it! :s
I will be seeing my parents at the weekend who have a Windows 10 PC with Edge and will be able to do some testing.
Hopefully it is just my PC, I will try a fresh local install of GP using InstantWP and then add my child theme files and see if I can replicate the issue.