I have an early 2013 iMac (1st gen of new design) that freezes and reboots itself about once a day. I can't pinpoint a consistent usage when it happens but I would say most frequently when either watching Netflix or using FCP X. Sometimes it will do it with simple things like browsing or using Adobe Lightroom. I've run the extensive hardware tests and found nothing. I've run memory tests and found nothing. This never happened before the Yosemite upgrade - although that could be a coincidence. Any clues? Anyone else seeing similar behavior?