It’s a iMac 27-inch (mid 2011) running Sierra that’s misbehaving. So I’ve run the Apple Hardware Test - no problems found. I’ve reset Pram and SMC. It doesn’t appear to be any Apps that’s causing it, as it may reboot without any Apps open. I’ve disconnected most peripherals attached to it… still no cure. (And some time ago I had a faulty AMD Radeon HD 6970M replaced under Apple warranty.) The only common thread when this crash/rebooting occurs is when it’s woken up after sleeping all night long. After that, it’s mostly stable for the rest of the day. So, all in, I haven’t a clue why this is happening. Any help or suggestions would be most appreciated.