see Ryzen - ArchWiki for suggestions… the core one I feel relevant for you may be adjusting “power idle control” in the BIOS
… for me, I am also thinking of applying a bit more Voltage to the CPU. The wiki suggested “Curve Optimization” +4 to 6 points, but I don’t want to enable PBO just for that… so I may try adjust Vcore or LLC (Load Line Calibration) instead… as that’s really central to my issue @ System auto-rebooted... mce: [Hardware Error] in dmesg related to CPU… but the topic where I also learned about “Power Idle Control”.
Also make sure you have an updated BIOS… one with hopefully at least AGESA 1.2.0.2 (for my motherboard, BIOS’s with beta AGESA 1.2.0.3b|c are available, but I generally wait for official/non-beta versions)
Click for a recanted story (in my own words) about why this happens
While searching for solutions, I read a lot of comments that “It’s rock stable on Windows, why not GNU/Linux?” And here is what some of my reading has revealed… which sounds true, and explains why some of these BIOS tweaks work.
In the past, CPU binning yielded stable non-OC’d CPU’s, idle was generally always stable, and it was a bit of a lottery on how well it would overclock; if at all.
Today, with the focus on higher clocks and boosting… the binning tends to yield CPU’s that hit the desired boosted (semi-OC) mark (at higher non-idle voltage), leaving users needing to tweak in some cases to get idle stable (when the voltage gets too low for your silicon’s “happy place”) … the new (inverse) lottery… yes, it’s opposite day! 
But I also feel that idle on Linux is far more idle than on windows (my average CPU load in conky is 2% with 70+ Firefox tabs and Thunderbird open). So much bloat, background processes, telemetry, etc, at play in Windows that… really how idle is idle on Windows? So it turns out that GNU/Linux allows your CPU to hit stages of lower volts/power than windows ever did/does… which in turn really tests out it’s low voltage/power tolerance; leading to BIOS adjustments to achieve full stability (at idle)… I know, crazy right? 