Ok I have to admit I'm lost. XMP profile is now OK. Most of the time, when I changed something in the Bios, it would reset it.
I have absolutely no idea why it's doing that. The only thing I changed though (while trying different things) was disabling CSM.
And guess what?
My PC has been working and booting normally (and quickly) since then.
I haven't decided to return my MB yet as a result. Memtest (done in Dram calculator) and OCCT reported no problem, it seems to be working normally.
I could leave it like that and see where it goes! ....
Or I could try to tighten ram timings a bit
)))
Oh, and I also flashed the bios again, I thought maybe something didn't go the way it should at first.
Even if it continues to work, I know I'll be flashing the bios again as soon as a new one comes out ^^