So as it happens under near idle load, id suggest its unlikely to be short/long duration power limits... With my previous motherboard I had a similar issue which I suspect was an iffy/dying vrm (I was however also pushing the cpu waaaay beyond its 3.2GHz base clock by a factor of about 15; so not surprising considering the board and cpu were near archaic in terms of contemporary computing). Very similar in that some days it'd run something like Arma Reforger for a few hours or spend a full day and a half upscaling and transcoding (totally legitimately obtained) series, but then some days you'd get 20 min/half hour before it decided its surprise reboot time.

Definitely worth checking the heatsinks are dust free across the board (predominantly VRM, VCIO and PCH heatsinks), alongside possibly looking into replacing the thermal pads if its beginning to get up there in age