Hi,
I have seen these white lines all the years I have been running FS9. I use Ground Environment Pro, Ultimate Terrain Europe and FS Genesis Mesh. The frame rate is set at 65 (Bob Holland advice). I can reduce the white lines by switching off the internet on my computer before starting up FS9.
Operating system Windows 7 64-bit Pro.
I think the computer (HP Z600) just needs time to update the scenery - I have a GeForce GTX 1050 graphics card and if I fly slow the lines only appear now and again. It's a shame that FS9 cannot use the 8 available processors on the computer. I should have bought a graphics card with more memory.
Happy flying!
David