Why does video on youtube keep freezing but the sound keeps going?

That sounds more like a codec issue. What OS are you running?

How can you check OS? I dont even know what that is

OS is the operating system. Windows XP, Windows Vista, Windows 7...

i got windows 7 64 bit

is this problem only with Youtube videos?
What about other on-line videos?
How about playing videos from your hard drive?
And which browser are you having problems with?
Did you try another browser?
What are your system specs?

Full Screen Youtube Freeze... Solution. (It worked for me, Hope this helps)

Screen refresh rate on monitor doesn't match graphic processor.
I tried updating the drivers, that didn't work, but I noticed that if i scrolled the mouse over the video it would correct it self so I tried running something on top of the full screen video. OSK (on screen keyboard). Now I still had to look at the task bar but this did fix the freezing. So I thought if it's not the driver and it's not the hardware it must be the settings, somewhere... and it was. The monitor was at 59hz, the GP was at 60hz. I set them both to 60 and youtube will now play in fullscreen with out a problem (Or a program running on top of it).
To find out if this is your problem type "video card" into the search bar in the start menu. then click "view the name of your video card" it should bring you to a page that says "screen resolution" at the top, find and click the Advanced Settings link. A small window comes up, find and click the monitor tab. this will show you the refresh rate of your monitor. the large tab for the graphic and media control panel will show you another refresh rate. mine was set at 60hz.
I matched these first at 60hz and checked youtube, It worked. then while writing this i checked the refresh rate and it has gone back to 59!? so i changed them both to 59, I applied it and hit ok. full screen-youtube worked again. but the settings are not staying the same. the first one I described is at 59 the second at 60. I'm going to keep messing with it.
If anyone knows how to make these setting stick please message me.
As of right now I'm not having any problems with youtube playing full screen.
I don't know what effect this will have when I plug in the HDMI to a tv but at least I'll know the problem if it doesn't work. And Knowing is half the battle!