@Papa Rob
Thanks for your comment.
Regarding the heatsink, what I made was pretty close to Der8auer, instaed of I used and modified the current heatsink to have a right fixture.
Regarding that point, thermal aspect was pretty nice. I think the problem is the motherboard itself.
Regarding the CPU point I don"t know. I fear that's a problem of generation. I have seen that on this kind of CPU you don"t win a lot.
Also this CPU has temperatures that swing a lot during use. More than 15°C .
-->This is clearly not the heat transfer or the big inertia of the cooler.
There is something internally !!
To finish the story, I moved my two configuration to a 9700Kf + Asrock Z390m-itx. Two PC in 4 hours (win included), driver & bios updated automatically ...
I'm not an Intel fan, but I love when this kind of things works smoothly
PS: we have two 3700x available ...