Youtube no longer support Safari 5.1.10 and OS X 10.6.8!!!

You can try thefeather beta, although there are disadvantages.

Changing Safari's user agent to IE9 via the Develop menu also works (or did when I tried) but that's a pain, since it needs doing for each tab/window.

Something is is afoot with yourt computer. I just watched YT vids minutes ago with 10.6.8 and Safari 5. Works fine here.

In Safari Preferences > Security tab, under Web content, is the "Enable Plug-ins" checkbox checked?

Adobe Flash up to date?

In http://www.youtube.com/html5 are you in the html5 trial?

Running any Safari Extensions? Any anti-virus softtare? Any "I'll Clean Your Mac" sofdtware? If y ouare not sure what is and isn't installed, please download and install this free utility:

http://www.etresoft.com/etrecheck

It is secure and written by one of our most valued members to allow users to show details of their computer's configuration in Apple Support Communities without revealing any sensitive personal data.

Run the program and click the "Copy report to clipboard" button when it displays the results. Then return here and paste the report into a response to your initial post. It can often show if any harmful files/programs are dragging down your performance.

Looks like it's an issue that's being worked on by Google... They apparently screwed up something.

They say they're working on a fix : https://productforums.google.com/forum/#!topic/youtube/nQ-eI0VeVXQ%5B1-25-true%5 D

Look for the post from ytLauren