I have a Intel Core i9-9900K Coffee Lake S, with a Noctua NH-D15S fan on it. In a ASUS ROG STRIX Z390-F GAMING motherboard. I got it about 18 months ago and tried to overclock it back then to the 5ghz which seems to be almost standard with that CPU. But it got unstable, no matter how much I tried. with no obvious reason. (like heat) So I gave up. But with playing various VR flight sims, I feel I could have use for the extra clock speed.
I followed the basic steps of the various guides, It's supposed to be quite easy really. So I don't know why it got unstable. It would crash within seconds some times, some times it would work for half an hour. It seemed random, didn't matter if I just looked blankly at the windows desktop or played a game.
Overclocking a 9900k @ 5ghz all cores(?) stable and keeping the temperature in check is really dependent on Silicon Lottery. The 9900K is a hot chip. I strongly recommend you get a 360 AIO cooler if you decide to Overclock to 5ghz. I also recommend this video: