We finally managed to solve the problem by creating a new video. We recreated the video that bugged (VI) in the same codec that the one which was correctly displayed (V3, codec indeo 4).
We think that there was a problem of compatibility between Direct RT and the graphic card of our PC (NVIDIA 8800 GTS SLI).
With this new video, with the good codec (indeo 4), there are no more bugs!
Thanks for your advices
Aurelie