My Ryzen 7 3700X desktop machine had random reboots when in idle.
Stress tests worked fine over many hours, no problem there.
There's a Gentoo Wiki entry that recommends to set idle=nomwait and processor.max_cstate=5.
For me, it did not work and neither did the tips from this kernel Bug report.
Those random reboots disappeared after I reset my BIOS to optimized factory defaults.
The only change I apply to those factory defaults is to apply the XMP profile of memory modules.
It turned out that, if the XMP settings were applied, the random reboot appeared again.
Luckily i stumbled over this linustechtips forum thread where someone said that power down mode needs to be disabled.
Someone else said that they fixed this issue by setting the RAM voltage to 1.36V instead of 1.35V.
Anyways, disabling power down mode did the trick for me.
This setting is quite hidden and was located under memory timing controls in my Asus BIOS.
Here are my hardware specs for reference:
OS: Arch Linux x86_64
Kernel: 5.14.7-arch1-1
Uptime: 7 hours, 22 mins
Packages: 1258 (pacman)
Shell: bash 5.1.8
Resolution: 3440x1440
DE: GNOME 40.4
WM: Mutter
WM Theme: Adwaita
Theme: Adwaita-dark [GTK2/3]
Icons: Adwaita [GTK2/3]
Terminal: alacritty
CPU: AMD Ryzen 7 3700X (16) @ 3.600GHz
GPU: AMD ATI Radeon RX 550 640SP / RX 560/560X
Memory: 5597MiB / 32024MiB
Thanks for this, just disabled power down mode and time to wait a little bit I guess. Just curious, what motherboard do you have?