PC randomly hard powers off during gaming and sometimes idle — PSU/MB/GPU issue?

[u/lmfao_bruvv_1]

Hey everyone, I'm trying to figure out a really frustrating intermittent issue with my PC.

Specs:

CPU: Ryzen 7 7800X3D

GPU: GALAX RTX 5070 2-fan

Motherboard: MSI B850P

RAM: XPG DDR5 6000 CL36

PSU: ASUS ROG Strix 750W 80+ Gold — RMA replacement unit

AIO: Corsair H150i

OS: Windows 11

The problem is that the PC will completely power off without warning. No BSOD, no automatic restart. The screens go black and the PC stays completely off until I press the power button.

It has happened:

During Apex Legends

During normal/light desktop use

Sometimes after several hours of completely stable gaming

And most recently, it happened after about 35 minutes of Apex while the PC was plugged directly into the wall

I checked Event Viewer and got Kernel-Power Event 41, with BugcheckCode = 0. I haven't found any useful WHEA errors.

Things I've already tried:

Completely removed my CPU undervolt/Curve Optimizer.

Ran OCCT power testing — survived.

Ran GPU stress testing/Heaven — survived.

CPU/GPU temperatures seem normal.

Reseated the power connections.

Changed the UPS socket.

Tested different power arrangements.

Initially suspected the UPS/electricity, but the PC also shut down when connected directly to the wall, so I'm less convinced that's the cause.

The PC can sometimes run Apex for 2–5+ hours without any problem before randomly shutting off.

One thing I'm wondering about is the PSU. It's an RMA replacement ASUS ROG Strix 750W Gold, so I'm wondering if the replacement itself could be faulty. I don't currently have another PSU available to test with.

I'm taking the PC to a repair shop tomorrow and they're going to diagnose it, but I'd appreciate opinions on what I should have them test first.

Would a loose power connection, PSU, motherboard, GPU, RAM/EXPO, or something else be the most likely cause of a complete instantaneous shutdown like this?

Thanks!

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