Just as a follow up, turns out the PSU was defective, so the place I bought it from replaced it. The new one had 3 separate PCIE cables, so maybe that helped too! But now the system is stable!!
Specs:
Mobo: Gigabyte A620M DS3H
CPU: Ryzen 5 7600X
GPU: XFX 7900 XTX
RAM: 2 x 16GB Vengeance 6000 MHz (EXPO ON/OFF still turns off)
CPU Cooler: DeepCool AK620
PSU: Seasonic B12 850W 80 Plus Bronze
PC shuts down almost instantly after starting stress test from AIDA64 and FurMark. Could this be...