Would you guys mind telling me what settings you're running (if you're on a Z390)? Possible I've got a dodgy 9900K, rather than a dodgy motherboard...
CPU Upgrade - Default
Enhanced Multi-Core - Enabled (although, up until just recently, it was set to disabled)
XMP - Disabled
I used to have XMP and CPU Upgrade enabled, but...opted to disable them to try to see if they caused the issue. I think the system does indeed feel more stable with them disabled/defaulted, but...hard to tell. I still haven't seen any issues for over a week now.
Either way, I don't think you have a dodge 9900k. I saw the exact same behavior before I upgraded to a 9900K processor. If anything, I would suspect a board problem over a processor problem, but... I'm still not convinced it's a 'bad board'. I've seen the same issue across multiple processors and motherboards now.