I have no idea if this has been posted earlier, but I am not in the mood to read through 42 pages of messages. Sorry.
First the problem.
Upper 1/3 - 2/3 of screen flickers black randomly. Even if not a showstopper, it's pretty annoying.
Then the solution ( at least in my case )
Completely reboot the computer, i.e. coldstart.
As it turns out, I use my MBP to play games. In this case WoW.
My powersaving is set on longer battery life, and it has always puzzled me, that despite this setting, WoW runs great, and left back of the computer gets extremely hot, indicating that 3D accelleration is running at full power.
And, when I close WoW down, and goes back to normal desktop operation, the screen often starts to flicker.
In my opinion, this is a problem with the video driver, maybe because some settings aren't fully restored then graphics are switched from 3D back to 2D.
Anyway, a complete coldstart always fixes it.