[xubuntu] flash/youtube performance? Issues with last plugin?

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http://ubuntuforums.org – Hi everyone! I'm a newbie in the linux world and have just installed Xubuntu 11.10 on an old laptop (Pentium II - 1,20GHz - 512 MB RAM). I thought initially that youtube, and hence flash, didn't work at all in firefox: back hole where the flash player should be. I've tried every possible installation with flash aid, then also reinstalled with " sudo apt-get install flashplugin-installer", then tested with youtube/html5, before I discovered that not all youtube videos had this black hole. Some videos worked, although the image was of bad quality, only 1 image every half second (Hardware)