Using Firefox and Chrome.
youtube wrking with HTML5 when flash plugin inactive.
but
youtube just stays still with black window, after loading the video, when using **flash plugin** to play it.

(why flash plugin needed isntead, if HTML wkring? HTML5 hangs quite long, the same video clip through the same internet connection working with no loading breaks on other devices (adnroid) suspect using flash plugin on them).

re-installed the plugin, exaclty the same problem

version flashplugin-installer 11.2.202.310ubuntu0.12.04.1
any suggestions? 
thanks