Youtube returns to low quality despite my settings?

Well, here I have a small problem that annoys me at the highest point, I have an account on youtube, I set preferences to always see the videos in high quality as I have a broadband connection, it records well settings but after a few days, bang default videos are low quality, it can happen when I restarted my PC . I wish it definitely keeps my setting.

I had the same problem, I solved by using firefox and greasemonkey

I already use Firefox browser, but thank you for a track

It is a addon for firefox.

https://addons.mozilla.org/en-US/firefox/addon/748

greasekonkey is installed when you go on userscript.

http://userscripts.org/

And you do a search on youtube hd

There are many. Jutilise most popular is:

http://userscripts.org/scripts/show/13333

you click install on the top right and then goes on youtube:)

Thank you pipe but by default this script not chosen the HD version, it does not read the default version of HD video, if I have to select it anyway every time I see no point in this script in GreaseMonkey.