The first frame of videos are blinking on pavlovia

URL of experiment: Pavlovia

Description of the problem: Hello, I am presenting two different videos on the right and left sides of the screen. But those are blinking at the beginning. When I run the experiment on different computers the problem might disappears sometimes.
I would be glad if you could help me about this topic. It is an important counfoundingwhatsapp web for my experiment, because it distracts the participants attention.