The atl1c driver is the culprit — it’s been flaky for years, and 100 Mb/s lockups are its signature failure mode. Things to try, roughly in order:

1. Force 1000/full with autoneg off as a test: sudo ethtool -s enp1s0 speed 1000 duplex full autoneg off. If it holds, at least you’ve confirmed it’s negotiation-related. (Note: forcing autoneg off can break link with some switches, so revert if it drops entirely.)
2. Disable PCIe ASPM — atl1c has known bugs with power management. Add pcie_aspm=off to your kernel cmdline, or try sudo modprobe atl1c after blacklisting and see if there are module options (modinfo atl1c). Also disable EEE if ethtool shows it: ethtool --set-eee enp1s0 eee off.
3. Check dmesg at the moment of the freeze — if you see atl1c: ... link is up 100 resets or DMA errors, that’s the driver. Kernel 6.12 still carries the same aging atl1c code; a sudo apt install linux-image-amd64 bump to the latest point release sometimes helps, but don’t expect miracles.
4. Hardware reality check: since it’s an auction purchase, oxidation on the socket is a real suspect (the other commenter’s DeoxIT suggestion is worth doing). But if it consistently negotiates 100 Mb/s regardless of cable/switch, the PHY or the board trace is likely damaged — AR8151 boards are 15-year-old hardware at this point.

Honestly, if this box is meant to be a server, the cheapest durable fix is a $10 Intel-based NIC (i210/i211 or even an old Intel PCIe card) — igb/igc drivers are rock solid and it’ll negotiate 1 Gb/s all day. That’s less time than you’ll spend debugging atl1c.