Remember reading somewhere on this forum about something similar where the culprit turned out to be using an older psu with the Ryzen. He also always got crashes when watching youtube during low load.
With older generation psu it could be fixed with BIOS setting for the Power Supply. Check you BIOS for "Power Supply setting" and set "Powers Supply Idle Control" to "Low Current Idle". Could be worth a try.
Just scanned my mobo bios manual for that setting. Don't see any power supply settings. If the 5800x is unstable will look harder. Should be in by end of week.
On my MSI MAG X570 Tomahawk it´s under OC - CPU Features.
I think I was wrong in my previous post because now I´m unsure if it was to be set to low current idle or normal. But if you find it try changing it and see if that does anything.