Hello, when the Yosemite beta came out, I downloaded it onto my late 2013 macbook pro. I have had no problems with it till now. After updating to the last update, when I try and do anything slightly heavy on my mac, it closes all my apps and logs me out. This means I can't download anything, I can't download the new yosemite update from the mac app store.. My mac has become useless! I need some help urgently as this computer is the only thing I have for school and it cost so much! Thank you!!