I had flashing at the login screen 100% of the time. That is fixed, but sometimes it still happens in the lobby. If I enter the game from a flashing lobby the game will flash too.
Clicking the boxes in the top right corner of the screen to 'restore down' and then 'maximize' always fixes it for me.
No problem with me now. There used to be a time a while ago when I got this flickering screen problem with Intel HD graphics card. The fix was really simple though. I just resized the windows, minimizing and maximizing it worked too.
...We've released a hotfix to revert back while we investigate the issue further. Close all open clients and relaunch the game and that should resolve interface scaling issues, and some launching issues for Intel chipset users as a bonus.
JagexDoom posted that on reddit. It probably worked, because my flashing login screen has returned.