when I enter utube I have sound but not picture. If I make a pop out to the video the I have picture( in the pop out)

what ever brouser I used when I entering to u tube I pick something to watch(a song).it is starting to loading the video but suddedly the load of picture is stopping after a few seconds and the loading of sound has no problem. as a result I have only sound but no images at all. the odd about all this is that when I make a pop out to the video there is no problem. I can watch with sound and picture. the same happens when I watch a video in the face book.I have no problem at all. all the above situation happening with all the videos....ps: when I picked mozzila and use it for the first time for entering u tube only the very first video played normally and then again I had the same problem...