DO NOT BOOT WITH KNOWN FAULTY RAM.

Sorry for shouting but that will lead to corruption and data loss. You really should wipe your system and start from scratch as the corruption won't just fix itself. I would restore important files from a backup and then destroy everything else.

To anyone else reading this, there's something you should know:

Memory errors don't always mean the memory itself (hardware RAM stick) is bad. It can also be a power issue (bad PSU, incorrect voltage set in the UEFI), compatibility, defective memory controller (CPU or motherboard), and more.

OP almost certainly has a bad stick, but it's worthwhile for anyone building a PC to run a slew of stress tests and diagnostics before using it for anything that matters.

I ran some random test (its passmark memtest86 v9.3 pro) on my medicat usb. Right now its 92% finished with 1070 errors. This just can’t be good :(

Not familiar with medicat. Are you saying memtest86 gave you 1070 errors ? Then one of RAM modules is faulty. Or is this about the hard disk and bad blocks ?

gentoo doesnt have systemd (I want to use Mullvad as my vpn and their app reqires it).

If I recall correctly it is technically possible to run Mullvad and OpenVPN manually without systemd for example on a SBC (pi4 etc.) as your LAN router, and feed it to your devices, but yeah this is a bit cumbersome.

Sounds like serious hardware problems (bad memory sounds highly likely if that's what memtest is telling you). Replace the faulty hardware before changing out any software, and before the badness "spreads"; you may already have corrupted a certain amount of the data / installed software on your disk by writing back data after the bad memory corrupted it, if you've been running on the broken hardware for that long.

Gentoo does have systemd, actually—package sys-apps/systemd—and there are optional sections in the install documents that explain how to go about using it as your primary init. It's an officially supported configuration, just not the default.

(But yeah, as for the main problem, sounds like hardware—RAM, your primary hard disk, or the disk controller on the mobo. Start with The Bleeding Obvious and make sure all cables are solid in their sockets and all the RAM is properly inserted.)

I’d do some compute tests on a usb live system. Something like y-cruncher for instance. Could be all kinds of things. Power supply could be flaky, gpu, cpu,motherboard, storage. best suggestion is process of elimination.

Try using it with a gui and see if you can crash it if you don’t have alternatives for example. Check the storage’s smart health. Check dmesg. Ssh in when a crash happens if possible.

FWIW I've also had memory issues with XMP.

Turns out that ASUS firmware is omega pepega and decided to go against AMD's specifications even for XMP profiles.

CLDO VDDP was stuck at the same voltage as SOC. Per AMD it has to be up to VSOC - 0.1V

So, after manually setting that, and other VDDP and VDDG voltages, it magically started working perfectly.

So do check voltages anyway even if you found a bad stick. Mine endured through the crappy firmware thanks to it being Samsung B-die.

Also check this for more info in general (I recommend this even if you won't OC, just the memtest alone is a huge section)

https://github.com/integralfx/MemTestHelper/blob/oc-guide/DDR4%20OC%20Guide.md

I tested with OCCT to find even more errors, so either do that in a mini windows environment or do one of the Linux tests to check memory some more. Memtest86+ isn't enough.