PC Freezes, reboots or shuts down ∽10m after coldboot

System: Ryzen 7 7800X3D, RX 7800 XT, Corsair RM850x 2024 PSU (replaced once already, same issue on old PSU too). B650-A Motherboard

PRE-DDU (before GPU driver reinstall):

- 4+ full unexpected shutdowns over 2.5 months. Event Viewer: Kernel-Power ID 41 each time, BugcheckCode 0, WHEABootErrorCount 0, no preceding WHEA/Display errors. Signature = instant total power loss, not a Windows crash.

- Occurred on two different PSUs and two different surge protectors, ruling those out individually.

- Most shutdowns happened within ~10 min of cold boot. One happened while using AMD Adrenalin recording/Instant Replay shortly after boot (also got stuck on "Saving..." once).

- Separately: PC freezes/stutters (no shutdown) reproducibly at the exact moment the apartment AC compressor kicks in, suggesting voltage sag on a shared circuit. Confirmed correlated multiple times.

- OCCT Power Supply stress test (CPU+GPU combined load, 40 min): zero errors.

- Panel is a Federal Pioneer Stab-Lok panel (known for breakers sometimes not tripping under fault). Landlord looped in an electrician; first suggestion was moving AC to a different outlet (not yet confirmed to be a different circuit).

POST-DDU (GPU driver clean reinstall):

- First recording test post-boot: no issues.

- Days later: brief 10-second screen freeze ~1 min after login while multiple startup apps were launching, accompanied by audio stutter/looping through headset. This time it was an AMD driver timeout (TDR) with the "submit report" prompt, i.e. a logged, recoverable driver event, not a silent power-loss shutdown.

Open questions:

- Whether the pre-DDU shutdowns and the AC-correlated freezes share a root cause (voltage sag hitting a PSU that's less stable in the first few minutes post-boot) vs. being two separate issues (electrical circuit + a flaky GPU/encoder driver).

- Post-DDU TDR event suggests driver instability may not be fully resolved, or may be a separate issue from the power-loss shutdowns entirely.

- Never had a WHEA-Logger entry in any event, so no component has ever self-reported a hardware fault.

Looking for input on whether this pattern points more toward: marginal PSU/motherboard power delivery during boot-time inrush, a still-unresolved GPU driver/encoder issue, or purely the AC/shared-circuit theory, and any further tests worth running.

UPS is also on the way, will see if issues persists.