When I upload the video from Fraps direcly to Youtube, the black bands are presents on the video but disappear when you switch to fullscreen !
That's because the black bands do not exist, as I have already said. When you switch to fullscreen, the aspect ratio of the rendering area is the aspect ratio of your screen. Since that matches the aspect ratio of the video, there is no empty space.
Anyway, glad you finally got it figured out.