For hardware issues. I'd unseat everything, including the CPU and memory, blast out the slots with compressed air, and reseat them again. Also check the cables with vometers, while wiggling them, to make sure there aren't any intermittent connection problems. If there are known questionable hardware items, they need to be swapped out one by one with known good items, to see if the problem persists. Also, one big culprit for these kind of problems is dual-channel memory. Try running with just one stick of memory, then switch to the other stick the next day, and see if your problem happens either time or both times. These are things for your Otousan/computer specialist to perform in your stead, if you feel unconfident in your disassembly/reassembly skills. On the software side, your computer specialist should also check to see what all your computer is doing at night while nobody is looking, such as automatic updates, attempts at hibernation/sleep mode, and the like, and looking at the event logs to see if a raft of errors are piling up in the background. Edit: Posts happened while I was typing all this mess. You have a hardware/wiring problem, for sure. It could be anywhere, since PSU, memory, and/or CPU problems can corrupt a boot sector/boot record in a hard-drive, in addition to the possibility that the hard drive or hard-drive cable is bad.