Random power offs & reboots

The PC is a fairly old custom built machine with an Asus P6T motherboard running Win 10/64 Home Build 1703. It has started powering off and rebooting itself at random times. When it does this it reboots 3 times, gets itself to the Recovery partition, and then shows the standard options that allow me to boot in Safe Mode with Networking.

The machine seems to run OK in Safe Mode with full Network support. But of course this is not acceptable for normal use. I don't understand why it should run OK in Safe Mode but randomly power itself off in Normal Mode, but my guess is there may be something wrong with the power supply. I can replace this pretty easily, but before I do this I'd be interested in any other ideas about why the machine should be randomly powering off and rebooting itself.

Thanks, I'll try a ram test and follow up with a stress test if that runs OK.

I ran the mdsched RAM test and it found no problems. The fact that the machine boots & runs fine in Safe Mode, but not in Normal Mode, makes me think there is something loaded at normal boot time that is causing the shutdowns. I've installed Process Monitor and looked at the Event Log but not found anything (yet) that looks bad. I'm thinking about using Autoruns to stop tasks that look like they aren't necessary, but not sure how to identify those.

Yup - tried all that: no help. The system runs only in Safe Mode. HD Sentinal said C: was in "perfect" condition after running a short test. The disk is a Samsung 840-EVO and seems to have no problems.

I've done the Clean Boot thing many times and it always reverts back to the Recovery Partition & Safe Mode. I've tried disabling all sorts of startup items using Autoruns but that doesn't help either. The System Reliability screen showed only a couple of "unscheduled" shutdowns, which is not new news.

I did get some log files from Process Monitor, but those logs are voluminous and I was not able to find anything that looked like an error. I also checked all the Window\Logs files and found nothing useful.

In Safe Mode I was able to make a Macrium backup OK, but it's not clear that reformatting C: and restoring that would make any difference. I still think the problem is some bug in a task loaded at boot time, but what it might be is still a mystery.

Some progress, but still not out of the woods:

After one of my previous Safe Mode restarts I discovered I had no Internet connection. After comparing Services.msc with my other Win10 system I noted that many services were disabled. So I went through the list and re-enabled everything that was running on the good system. That got my Internet back so I shut the Safe Mode machine off for the night.

This morning it booted into Normal Mode OK. So I downloaded the Prime95 stress test program and ran it a few times. Each time the system powered off after a while and then rebooted itself. I always came back in Normal Mode, which was a major improvement.

Now I just have to digure out why the system is dropping power. The Event Log & Reliability Monitor simply say "The previous shutdown was unexpected" - not particularly helpful. I've got OpenHardwareMonitor but it doesn't seem to create a real-time log.