Strange crash-reboot issue

[Sweeflyboy]

I've just recently bought myself a second-hand PSU, CPU, MOBO, RAM combo.
It performs very well and has practically no stability issues in stress tests. However, I can't seem to load a single game without it crashing (Acts as if I've pressed the reset/reboot button on front panel). Sometimes it crashes on main menu, sometimes after several minutes of gaming. 
I'd write it off as bad PSU, but in games my CPU utilization is at around 20-40%. I've stress tested CPU and GPU together and had both reach 100% (obviously drawing more power than at 40%) without any crashes.
Temps are fine in games (60-70 on GPU and CPU).
Benchmarks/Stress Tests:
It has crashed only once in MSI Kombustor, could have been a fluke. (preset MSI-01 if it matters. Did not crash in the FurMark preset).
It crashes consistently (usually immediately after loading in) in Unigine Heaven on DX11 and DX9, and sporadically (usually after several minutes) when on OpenGL.
Games:
It crashes consistently on War Thunder on max settings and only sporadically when on lower settings.
It crashes fairly consistently on Hired Ops
It crashes fairly sporadically in Enlisted
It crashes *very* sporadically in COD Warzone (usually after 10 or more minutes)

I've each ram stick individually. Same issue.
I've tried turning off automatic restart on system failure, to no avail. 
I've tried turning down the power limits for GPU (seemed to last longer but could be placebo) and CPU(absolutely no effect, if anything it made it worse. Again, possible placebo)
I've used a completely separate Windows 11 installation, same issue.
It also seems to last longer if the PC has been on but idle for a bit, like if I was fiddling in BIOS or similar.
Once it crashes once, it seem to crash much sooner upon reboot than it would otherwise (Made me think it was PSU overheating, but the PSU fan doesn't even ramp up)
All power cables are definitely seated correctly.
Hope I've included all necessary info.
Spec list:
CPU: Intel i7 4790K
GPU: AORUS RX570 4gb
RAM: 16gb DDR3 (4x4gb)
MOBO: ASRock Z97 Pro4
PSU: Corsair VS550
OS: Win10 (tried Win11 as well; see above)

EDIT: I've used COD Warzone for about 3 hours now without issue. Since it is DX12 and all the other games are DX11 (to the best of my knowledge anyway) this makes me suspicious it's a direct-X issue.