Youtube freezes my Mac - All browsers on OS X

Hello,

This question has probably been answered in bits and pieces before.

I am experiencing a peculiar problem since I installed the latest OS X 10.10.5 Update on my MBR Pro Late 2012

The first whiff of a problem that I noticed was post the restart for this update my screen went black an inch into the OS X start-up progress bar.

Another restart, and I did not see this issue again (until later..)

Any YouTube video I play on Safari, Mozilla & Chrome freezes up my Mac and I have no option but to do a hard restart. I have also noted that this happens only when I am signed into my Google account. The videos are playing fine when i am not signed in. All other HD videos on my mac as well as on other websites (Netflix, Prime, etc..) are working fine.

I figured this may be a problem with the latest update and I used my hard disk to restore the Mac to a previously saved version thru Time Machine.

Here I noticed that during the restore, right from the time I had to use Command (⌘) and R, my screen went black again.

The restore did not solve the Youtube issue, my computer froze again and again.

These problems first cropped up since Aug 19, 2015 - the day I updated to 10.10.5, including the frequent black screen during boot-up.

Can someone please advice how to resolve this issue? Is it a bug?

Thanks for your time!

Debo.