I have a 9900K and I am trying to dial it in, so I run into similar situation but note my CPU can run 5.2GHz just fine. However doing poorly and a few things I am working on is to have a base to test against myself. Example is; Download Realbench R15, HWiNF64, and OCCT for now.
1.) Reset bios using F5 and save.
2.) Run 2 or 3 R15 test and record your scores plus make sure you have HWiNFO64 Open to record voltages and temps. If by any chance you run into the issue it will not load just launch it as soon as you enter your desktop.
3.) Once you have your test do some manual setup without limitations. Example of those settings put the cpu all cores to x47, set voltage to 1.3v, set cpu LLC level to 5, keep ram at auto or 2133MHz, and test. My understanding is you should be able to get on a 9900K @ 4.7GHz a score of 2000cb.
4.) Also check your Windows 10 Power Plan and choose High Performance or Ultra Performance for High End Systems then restart your pc and test. I have notice that by itself helped my score but is not there yet.
Note I don't know for sure but I was told a stock 9900K should be hitting 2034cb on R15 stock. Hopefully that give you something to work with as a base and temps stay below 90C my monitoring your cpu during test. Good luck anything else I end up finding I will try to share here with you. Intel 9900K cpus are amazing cpus and they work great depending on what you are using it for.
=) I am also learning so hang in there.
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