Thanks for the responses so far.
I did notice that you guys take the cooling a few steps further and that is fine.
My fans are running at max (1500rpm).
Still doesn't explain why some are able to run it on 5Ghz with 1,25V.
i9 9900X @ 5Ghz 1.25V HT On and 5.2Ghz 1.32V HT On / Thermal: Greezly Cryonaut / ROG Rampage VI Extreme
Separate [CPU+MB] watercooling loop = 3x TFC XChanger 360 (60mm thickness) with 18x 120mm fans + D5 Vario
Last night I did run Cinebench R20 for another 2h and crashed after 15 minutes multiple times.
So I took the multiplicator down to 44 (BCLK is 100) at 1,240V and it did run through.
I am aware that chips tend to differ, but that much?
On stock speeds, with just the default Asus optimized overclocking active, it runs on 4,1Ghz all Cores with 1,13 to 1,17V on the cores.
I am starting to wonder if those 300Mhz are really worth it.
I also noticed that one of my cores is more than 15° colder than the others.
My only reference is my old 5960X that I overclocked to 4,2Ghz with an NH-D15 and that was running on 66 to 70° with 1,350V.