When I am watching videos on youtube and toggle the screen to full screen or switch back from full screen to normal mode, the video stops and sometimes stops loading or starts loading from the start again. I installed ubuntu-restricted-extras so I have flash installed.
There's a fix posted here. It seems to have done the trick for me, but I'm not 100% sure it's the same problem for you. This is a problem with Flash and its hardware acceleration, not YouTube itself.
In case the above site is unavailable, here's the fix:
sudo mkdir /etc/adobe echo "OverrideGPUValidation = 1" | sudo tee -a /etc/adobe/mms.cfg
This happens to me too. I think it has to do with changes in Youtube's player itself. In the normal mode, the default player quality is usually 360p. However when we change to fullscreen, it automatically changes the video quality to 480p and starts loading for that quality. That's why it seems to restart. I can't check right now, but I think it is possible to change these default settings in our user preferences in Youtube. I don't think the problem is Ubuntu or Linux specific (but perhaps it is related to the new flashplugin available for Linux now, but I am not sure).