Problem with video playback

I have a recurring problem with video playback. This effects sites like youtube and all other souces of online video.

When the system is first booted up video playback and sound is fine and then at some random momen,t which can be after several hours video playback is at about half speed (though it is still smooth and not jerky)and there is no sound. (Indeed the problem seems to prevent any sound file being played) A system reboot rectifys the problem until the next time. I have been searching for a solution for weeks now but just cannot find one.

Any suggestions would be appreciated

Yes, all flash palyer updates are installed as they become available

No, Firefox 3.6.11

Yes, it makes no difference

I would check what processes might be spiking your CPU usage during the time when this starts happening. It could be something just going haywire.

CPU useage meter at 8% Ram at 37%, so clealry this is not an issue

Do you have any third party codec packages installed?