I think I have found a fix after hours of testing 
Part of me thinks only the 4th step really fixed the issue, so feel free to jump to that. But just in case, I will give you the exact 4 things I’ve done to solve this.
- First download this free add on from flightsim.to. Make sure you follow their instructions for installation, video here.
- Turn rolling cache off, delete it and apply and save. The path is below:
Options > General Options > Data > Rolling Cache Settings - Turn it back on and give it a size limit of 50GB. Apply and save. You should then get a screen saying it is being configured.
For those who say they’ve done this, please make sure you get a loading screen. Earlier today I tried doing this with the rolling cache and when I came back a second time, rolling cache was off again! But I never got a loading screen the first time round. When I did it this time, there was a loading screen confirming something was happening and now the settings have remained.
- I then changed two settings under graphics .
Options > General Options > Graphics
I moved the Terrain Level Of Detail slider (Terrain LOD) to 100 and I set Off Screen Terrain Pre Caching to High.