Buffering type of problem when trying to play youtube-video on Internet Explorer 10 & Firefox

When I try to play youtube-video on Internet Explorer, I find out that it stops always at 0:15 regardless of video and it might continue after a while but still it stops now and then and after several times of doing this it says "An error occurred, please try again later. Learn more.
I have tried to join HTML5 Trial, but after clicking first time it doesn't do anything and after second clicking it gives me a page where it says;

405. That’s an error.
The request method MTIyPOST is inappropriate for the URL /html5. That’s all we know.

And when I now have deleted cache of Fire fox(4.?) and cookies and tried to watch youtube-videos on Fire fox, it has mostly worked except few abrupt pauses without reason at 0:19, 1:15, 1:44, 2:29, 4:28, 4:55( Quality: 360p)(http://www.youtube.com/watch?v=TItyYhfwClM&list=PL3HSVTBo3r6hENakA7brBQUMI52hVJxeu). And now this next youtube-video(http://www.youtube.com/watch?v=qK8TDp0k3Wo&list=PL3HSVTBo3r6hENakA7brBQUMI52hVJxeu) stops at 1:33 something and proceeds to the next one(http://www.youtube.com/watch?v=3ao9eQrbZvo&list=PL3HSVTBo3r6hENakA7brBQUMI52hVJxeu&index=8) which stops at 0:14, 0:29, 1:14, etc. At the beginning it looked like playing youtube-videos without problem on firefox, but now it starts to behave like on IE.

When I reset all settings of IE and restarted computer, youtube-video still acts same way as before resetting and also regarding HTML5, it makes same error 405... regardless of web browser.
All other videos except youtube- videos, work on IE and Firefox.