Why will Youtube videos freeze but the audio carry on?

I'm having trouble with my new Macbook Air.

I went to watch a programme on BBC iPlayer and it said that I needed to download Adobe Flash Player. I followed the instructions to download and install Flash Player and it said that it was successful. Again, I went onto the BBC iPlayer website again and it still says that I need to install Adobe Flash Player.

Since installing the Flash Player, when I have gone to watch a Youtube video the video itself stops when I stop using the mouse but the audio continues. However, if I continuously move the mouse the video will play normally.

I have no idea why this is happening so any help would be much appreciated.

Thank you!