Question GPU no display. MOBO display working fine. Trying to narrow down issue.

Oct 1, 2021
CPU - Intel i7-4790K 4x 4.0Ghz/8MB L3 Cache
Mobo - ASUS Z97-K
GPU - NVIDIA GeForce GTX 780 3GB Single
PSU - 850W Thermaltake 80 PLUS Bronze

All of these components are about 8 years old. I'm not that knowledgeable about computers but know enough to do simple troubleshooting. I'm getting no display from my GPU HDMI port. The MOBO port is working fine with the GPU taken out.

I've taken the GPU to 2 different local repair stores to diagnose it. The first guy quickly plugged it into another PC and told me "No good. No input signal." Didn't fully trust his diagnosis. 1 day elapsed with it being unplugged. Brought it to another guy. Saw with my own eyes that it was working in another PC. He told me the issue might be my BIOS having to be reset due to a power surge. I promptly went home and removed the MOBO battery to reset the BIOS. Plugged the GPU back in. BAM! It's working again. I have display with my GPU. I thought everything was all good. 4 hours later, my PC randomly shuts off out of nowhere and reboots itself after about 20 seconds. "Uh oh", I thought. Then for the rest of the night it was still fine. The following day the same issue happens. Just shuts down, this time it doesn't reboot itself. I manually go to turn it back on, the PC's components turn on but I now have the issue of no display again. I tried resetting the BIOS again. The BIOS reset shows but I get no display when it tries to boot up the computer into the login screen. I've tried unplugging the RAM sticks and putting them in different slots. I just did this today and put the GPU back in. And the computer kept turning on for 3 seconds and then turning off again. Unplugged the HDMI cable, the computer continued running without turning off. Plugged the HDMI cable with it running, and it shut off again. The computer still seemingly remains to be fine with the GPU removed.

Based on these issues, I'm assuming it's one of these three issues.
  1. GPU is slowly dying but was briefly working again after being unplugged. Thoughts?
  2. PSU is faulty and not providing enough power to GPU - fans and other components run with GPU plugged in, everything is seemingly running fine with GPU removed. I'm starting to think this might be the issue due to the PC abruptly shutting off twice.
  3. Driver issues/Malware/Corrupt files
Other issues could be the MOBO itself or the CPU. But judging by it working fine without the GPU being plugged in, I want to say these are less likely than the above three. If anyone reading this that is more knowledgeable than me about computers could perhaps provide some insight on my problem, it would be extremely appreciated. I'm curious to see if there are things I may have overlooked or if there are other things I could try. Thank you!