PC Freezing

Hi everyone! I hope this is the right subreddit to ask this. My pc has been giving trouble for a couple of months now.

At first, the screens would go black while the pc was still on and I would be forced to shut it down. After a lot of testing (checking wires, testing the screens and pc hardware in another pc), I thought it was the gpu. It was the oldest part in my pc (2015). I got another one (a barely used secondhand one due to crazy gpu prices now), and all seemed well for a few weeks. But it has started again.

My pc would just freeze, screens go black, the sound would stop, etc, but the fans and lights stay on. It happens at random intervals, sometimes a few times a day and other times once a week. I can't pinpoint a specific activity triggering it. It's happened while gaming, watching Netflix, working on a Word document. Sometimes the screens would show strange static colors. The temperatures are good but to be safe I added an extra fan which hasn't changed anything.

My pc specs: Processor: Intel Core i5 2.9GHz (2 years old) RAM: 16GB Ripjaw (2 years old) Motherboard: Gigabyte Z390 UD (2 years old) PSU: Super Flower Silver 750W (4 months old) GPU: Nvidia GTX 780TI (unknown age, but the friend I bought it from barely used it) I have two screens and two hard drives, all under 3 years of use.

What part could be the problem so I can test it in another pc? Most pieces are still under warranty.

Thank you!