I can't remove some elements in a slide

Hello everybody,

I can't remove some elements in my slide. They don't appears during all my slide, just sometime at few frame.

My layers are not locked, I don't understand where these elements are.

I check : they are not un my in my backgroud's picture.

I join some pictures (with some part blurred) to show my problem.

Element to remove :

It appear at the first frame (and two more near the slide's end) :

When the slide play, the element disappear (except 2 or 3 frames) :

My trouble : we can see these element (like a flash) when we watch the video.

Thanks a lot !