RX 5700 XT intermittently prevents POST, AMD Crash Defender error and Code 31

RX 5700 XT causing intermittent no POST / no video issues

PC:

-Ryzen 5 5600GT

-ASUS Prime A520M-K

-RX 5700 XT ASUS ROG Strix (used)

-Antec CSK550 PSU

Symptoms:

-PC works perfectly on integrated graphics.

-If the RX 5700 XT is installed and powered, sometimes the PC does not POST.

When the issue happens:

* No video output.

* Keyboard does not power on.

* GPU RGB may light up, but system never reaches BIOS.

* If I disconnect the GPU power connectors, the PC boots normally using integrated graphics.

Additional events:

* I previously received an AMD Crash Defender error.

* During normal use, both displays froze, the GPU RGB turned off, and Windows continued running on the integrated graphics.

* Device Manager then showed the RX 5700 XT with a warning icon and Code 31.

* After rebooting, the system again failed to POST with the GPU powered.

Things already tried:

* DDU driver removal.

* BIOS defaults loaded.

* UEFI settings checked.

* Reseating the GPU.

* Power drain (residual power discharge).

At this point I'm trying to determine whether this is:

* a failing RX 5700 XT,

* a PSU issue,

* or a VBIOS / motherboard compatibility issue.

Unfortunately I don't currently have access to another PC to test the GPU.

Any ideas or similar experiences?