The title says it all. I've launched Console and looked at the entries around reboot time but I'm not seeing anything suspicious. Maybe I'm looking at the wrong logs? Where's the best place to look? Just launch Console and look or should I open a different log from the sidebar? What am I looking for? I've searched for "panic," "reboot," "restart," and for long strings of repeated entries. This is maddening. It's happening as many as four or five times per day. Update (12/14): It looks like it's probably hardware: I booted from Snow Leopard again and let the computer run for a couple of days and it rebooted spontaneously under Snow Leopard. I guess it was a coincidence that it happened around tech same time as the Yosemite upgrade. I guess I'm looking for a hardware problem now.