If you see the printscreen, you can see that it work really well, but temporaly. All the CPU work until the middle of the render.
This is the same composition and there is nothing new that appears, and then their performance decreases without reason.
What can causes instability?
I did some tests by removing a .mov video file (with transparency) in the project and rendering was done with full power !!!
At least in After Effects that's OK, it does not explain the poor performance in Premiere. A friend of mine told me that the GTX (gamers cards) are more volatile than professionnal cards. This would explain why some times it is better than other times. Is that true?
Tanx for your support