I know this was a while ago but I wanted to share my experience as maybe it will help someone else. First, know that you are not alone. There are several people that have had this weird behavior. No one knows exactly what causes it and there is no definite answer or solution to it that I know of. But generally people point to the chip, the motherboard, drivers, overclocking, and power management as the issue. It may very well be some combination of those things.
But the reboot of the Ryzen systems when at idle is really frustrating and very hard to diagnose.
I don't have an explanation for it but I have found that resetting Windows makes it go away for a while. That would indicate some driver conflict. I had this issue with my older Ryzen 7 1700X system, and I'm having it now with my new Ryzen 5 5600G system. Maddening! The new system ran just fantastic for two weeks and then the reboots of the system every time I walked away for a few minutes would happen again.
I tried all the suggestions: turning off overclocking, disabling all the overclock gizmos in the BIOS, turning off the sleep settings in Windows. Nothing works. Only resetting Windows did it. But in the last time I did this, I noticed something...the behavior seemed to start on the new system shortly after I'd done a big rush of installs of AMD drivers (I just went to the site and pulled down everything that was applicable to my chip). So I assume there's something in the chipset driver or the video drivers that might be causing an issue.
Now that I've reset Windows, the system is back to it's newborn self and is stable and performing great, and my plan is to just leave it with the default drivers that Windows installed. I may gradually experiment with adding the latest chipset driver later, or the Radeon drivers, but just one at a time. I was so frustrated when the issue appeared on my new system that I was ready to throw the chip out the window and go buy Intel...but I'm glad I could get the system back in order and will be more careful with the driver updates.
The Windows reset does come with some hassles; you'll have to reinstall your apps and rebuild or import all your browser settings, but it's worth it to get some system stability. The Ryzen is a great chip as long as it's stable, right?
