My PC freezes and runs slowly sometimes. I suspect the motherboard or network card. Some time

Years ago, I built my brother a PC with spare parts I had lying around. It had an i5 1155 processor, a Gigabyte H61 motherboard, 8GB of RAM, and an OEM FSP 350W power supply. The motherboard worked fine, but the LAN port was broken. So I replaced it with a TP-Link TG-3468 V4 PCIe adapter.

Some time later, he told me the PC was running slowly. It took ages to boot, do anything, or shut down.

I replaced it with a better one. Back then, I tested it, and the power supply burned out a SATA connector on a hard drive I was using. So I threw that power supply away.

The other day, I salvaged the motherboard and installed it in a mini-tower case. It worked fine the first day, but not the second. When using the internet, the system sometimes freezes. It's like the hard drive or CPU is running at 100%. Sometimes, after waiting a few minutes, it lets you use the PC again. But if you keep using it, everything becomes incredibly slow. It takes 5 minutes to boot up, and then it won't shut down.

I removed the network card, since I saw that other people were having problems with that TP-Link card. But if I connected anything to the rear USB ports with the PC on, the PC would freeze, and the USB ports would stop supplying power.

In the end, I installed another H61 motherboard I had lying around, and everything is working perfectly. But it wasn't detecting the SSD, and I had to use a different SATA cable. I don't think I used the same SATA cable in the other case, or who knows.

What could it have been? A faulty motherboard? A damaged SATA cable, which is why the PC was freezing? Or was it finally the network card?