I was also experiencing something like this. I would open lots of tabs, and at some point the machine would freeze and I would need to do a hard reboot. This got worse after installing Yosemite, where I could rarely go a day without having to reboot my system.

I was about to take my laptop in to see if it was a hardware issue when a coworker pointed me to a forum where someone suggested turning off automatic graphics switching. I did that about two weeks ago, and since then I haven't had a single occurrence of the issue. You may want to try the same thing to see if it helps.