I bought a used 2009 Mac Pro a few weeks ago. I performed a clean installation of Yosemite and began using it as my primary machine. It seemed to work perfectly, for about a week, before I ran into this issue: As I was browsing the internet, the cursor froze, I could get no response from the keyboard, and then after about 30 seconds, the machine powered off and restarted. The startup chime sounded garbled, and I was greeted with this: and, upon booting into OSX, this: (This second one, perhaps not exactly this message, I'm going from memory here) I chose "Open", and everything came back up, including the browser, which I used immediately to begin researching the problem. One or two minutes later, as I was reading this Apple support page, the problem recurred. This time, however, I did not receive the same "Your computer restarted..." message; it went straight to the "progress bar" OS loading screen, but the bar only moved slightly and then stopped for several minutes. I hard restarted the machine, and now could get no display at all. I powered it off and unplugged it, thinking I would troubleshoot it the following day...which I set about to do, but now could not reproduce the problem. It started immediately up without issue. I ran ASD on it and everything* passed. Hoping it was a random, one-time thing, I continued to use the computer...for about a week, before it happened again. Same situation, for the most part. Slight differences (no "Your computer restarted..." message, no boot into OSX at all immediately after the first restart, no panic log that I could find), but essentially the same: it froze, restarted, hung on the loading bar screen, I hard restarted, it would not display at all, I let it sit for a few days, and it booted up again, no problem. I'm at a loss; please help me figure out what's going on here. Following is the kernel panic log from the first event, I can't seem to find any from the subsequent events: *everything except for something to do with the Ethernet MAC address, which I haven't quite figured out; I suspect may be a false positive, and don't believe is relevant here. If you think it is, I'll provide the details.