Yosemite (> 10.10.x) randomly freezes and crashes on my MacBook Pro 15" Early 2011
I had the same problem since a couple of days. Here's what I've done to (apparently) fix it:

Created another Administrator account. Logged onto that account. Removed DropBox, Skype, Viber, Evernote.
Why? Those where the only applications my new account did not run, and my new account never experienced any crashes.

I then got back to my primary account and downloaded the latest version of Skype (7.1) and Viber. I am now running Skype and Viber as before. I had no use for DropBox or Evernote anyway.

I used to have crashes shortly after restarting. I am now on my second hour with a solid machine. I don't know if this will stay so, but at least you can see if it is any help to you.