Have you ever get a problem like this?

I certainly don't know where to start looking for... it's so strange...

...sometimes, when I'm running around, I noticed the level begin
to flicker, then I found exactly the point where the problem happens.

If I stand at this place the flicker happens very fast and I took screenshots
to show how it looks.... it seems a lot of polys get missed by some reason ...

Any idea of what could cause this?