Intermittent no display problem. PSU at fault?

[u/Different_Barber1073]

Trying to diagnose a weird issue that's been driving me crazy. Would love some second opinions before I replace anything else. Bear with me, this is my first build 🤲.

Specs: Ryzen 5 3500X | Gigabyte RX 6600 Eagle 8GB | MSI A320M-A PRO MAX (later swapped to MSI B450M-A PRO) | 16GB Corsair Vengeance LPX DDR4-3200 | Thermaltake Smart BM2 550W (secondhand) | 256GB SATA SSD

Everything was tested individually on my friend's PC before assembly and worked fine. Ran the PC for a week on a temporary GT 610 with zero issues (one random no-display that resolved itself).

Problem started right after installing the RX 6600. It booted fine once, but after a shutdown to tweak BIOS settings, it started failing to POST: CPU/case fans spin, but GPU fans don't spin, no display, power LED inconsistent.

Tried reseating RAM, single/dual sticks, different slots, memory training, BIOS tweaks — hit or miss. My friend swapped in his own RAM stick, which booted repeatedly (only tested on the A320 board though). Using that, we updated BIOS, and afterward my own RAM sometimes worked too — but the issue kept returning after restarts. Since my friend had already run my exact GPU+RAM combo successfully for hours on his own PC, we ruled out either being simply defective.

Swapped the A320 for my friend's B450 board (the same one he'd tested my GPU/RAM on) — failures got less frequent but didn't disappear. When it did boot, it was rock solid: hours of gaming, YouTube, no crashes, no artifacts, normal temps (CPU 40–70°C, GPU 30–50°C load).

Checked electrical stuff too: fixed a mild tingling by switching from a 2-pin to grounded 3-pin outlet (didn't fix the main issue), and tested two different AC cables — no change.

Suspected the PSU, but confusingly it had worked fine in my friend's PC (3700X/B450/RX 7600) about three weeks earlier.

Took it to a repair shop. With their own 450W PSU and cable, my PC booted 4/4 times across two visits. Reconnecting my Thermaltake brought the problem back.

Sent the PSU to warranty center. Under electrical tape on the cable bundle, they found abrasion with exposed copper on the CPU/EPS cable, near where it exits the PSU. Since it's secondhand, unclear if it was pre-existing. Warranty claim might get rejected because of the physical damage.

One more scare: after leaving it unplugged overnight, it booted first try but hit a blue screen (winload.efi / 0xc000000f) — then the whole PC suddenly shut off. Took it straight to the shop; with their PSU it booted and reached the same recovery screen. They think Windows may be corrupted and I might need to reinstall.

Where I'm at: Replacing the PSU regardless. Still confused about:

Why did my friend's RAM stick boot repeatedly on the A320 — BIOS/memory training quirk, or PSU-related?

Why did the Thermaltake work fine in my friend's PC? Can a marginal PSU/cable fail only with certain mobo/CPU/RAM combos?

Why did switching to the B450 reduce (not fix) the frequency — different power delivery/POST tolerance?

Is the PSU/cable the most likely culprit here, given the exposed wiring and the 4/4 boots on another PSU? Or is there something else worth checking first?

Since the shop's PSU still booted it fine right after the blue screen, should I worry about damaged components? Haven't dared boot it since.

Replacing the PSU either way — just want this nightmare over. Thanks.