youtube videos repeatedly pause in firefox ubuntu karmic
 

I'm using Ubuntu karmic 9.10. Firefox works great on most internet sites but when I play a youtube video the audio keeps pausing for a second or two, the video only plays as a series of still shots and the whole firefox window keeps greying out.
How can I fix this aggravating problem?

Perhaps the following could provide you with a solution: how-to-fix-youtube-stuttering-in-firefox

youtube stuttering link didn't fix the problem
 

Thanks for the suggestion Forubu but it didn't fix the problem.
I've since tried other fixes to no avail as follows:
- I tried Dooble & Konqueror browsers but had the same problem, suggesting the problem may be in Ubuntu itself
- I googled the problem and followed the advise on other forums & web-sites, including updating Adobe Flash but theses didn't help either.
- Oddly, some youtube videos play fine and then others don't.

I'm open for more suggestions.