My computer either locks up or crashes when I am playing games on my homemade pc!?

I made this computer in June, and all was fine until two weeks ago when my computer randomly crashed during a game of Smite. This happened during a high stress moment where a lot of abilities went off, so I just wrote it off. Then, two days later, it happened 3 times in a span of 3 hours, one of them while just browsing the web and the other two on smite again. It didn't happen for another week, and then it started crashing on Rainbow Six Siege and locked up on Killing Floor for two minutes, then restarted after and input all the PC commands like CTRLALTDLT, both at times where I died. It stopped happening on Smite, not happening sense the day of three crashes, and I've played R6 Siege and Killing Floor before without problems, so what could be the problem now? Also sorry I don't my specs list at the moment, I would ask my friend who gave me the list but he isn't available right now.