Worked fine for 10 days, then began restarting itself 1-3 minutes after boot (no BSOD)

CPU: AMD Ryzen 5 3600 MOBO: Gigabyte B550M Aorus Pro RAM: Corsair Vengeance 2x8GB DDR4 2400MHz GPU: EVGA GeForce GTX 1660 Super SSD: WD Blue SN550 1TB M2 NVME PSU: Seasonic Focus GX 650W 80+ Gold Networking: TPLink AC1200 PCIe wi-fi card

Problem: built my computer 10 days ago and worked great! Starting yesterday, a problem: after booting to Win10, the computer restarts, usually after 1-3 minutes. No BSOD, just sudden shut down and reboot. Has run for 20-30 min a couple of times.

Runs with no problem: in BIOS, in safe mode, and while doing memtest.

In my logs, it's throwing an error:

Fatal Hardware Error Processor Core Cache Hierarchy Error WHEA Logger Event ID 18

Troubleshooting so far (in rough order): \- checked fans, including PSU fan: all running fine \- thermals: cpu spikes to 65 on startup, then idles in low 40s. runs in the mid 30s in BIOS. \- completely reinstalled windows and did updates. NB: installed fine, but then encountered the problem immediately, while doing all the updates (so it's unlikely to be, say, a GPU driver issue). installed AMD driver, NVidia driver, chrome, some diagnostics \- runs 95 when running p95 stress test. gpu is in the low 40s \- wd diagnostics turned up no errors w SSD \- memtest (4 cycles): 0 errors \- removed GPU and physically inspected front of MOBO: no obvious scorch marks or blown/bulging capacitors \- ran in normal mode with GPU driver disabled (problem still showed up)

These steps point away from: GPU, RAM, SSD, windows 10 install, thermals

Process of Elim & googling seem to point to CPU, MOBO or both. Is that a fair assessment? Are there any other things to check? Any ways to figure this out? Or should I just try to return both components for a replacement?