Based upon what I've read on this
page, it seems like the best two things to try are (directly copied from the page) :1.
From the Safari Menu Bar click Safari/Preferences then select the Advanced tab. At the bottom of that pane select: Show Develop Menu in the Menu Bar.
Now click Develop/User Agent and try Safari 4.0.5 when streaming YouTube content.
Note.. if you relaunch Safari, it will reset to the default Safari 52.
Try repairing disk permissions...
Launch Disk Utility. (Applications/Utilities) Select MacintoshHD in the panel on the left, select the FirstAid tab. Click: Repair Disk Permissions. When it's finished from the Menu Bar, Quit Disk Utility and restart your Mac. If you see a long list of "messages" in the permissions window, it's ok. That can be ignored. As long as you see, "Permissions Repair Complete" when it's finished... you're done. Quit Disk Utility and restart your Mac.
Hope this helps.