PSU cords?

I built my computer last spring, and seeing as it was my first build, I wasn't surprised I would have problems with it. By the time summer came, my computer would overheat frequently and shut off. I bought more fans for it and an aftermarket for the CPU, but it didn't work and continued to overheat. Summer was soon over and by the time cooler weather came, I didn't have anymore overheating issues.

Now a year later and summer in progress, my computer still continues to overheat and shut off. I tried orienting the fans many different ways, but it still shuts off. Yesterday, after the usual overheating + shut down, I tried to boot it, but the lights and fan would only turn on for a split second, then off. After a while, I had a spare PSU cord and just decided to swap it and it worked. My original PSU cord worked as well after booting correctly. Today after my computer shut off, it would again turn on for a split second, then off. I tried to swap PSU cords, as it worked yesterday, but didn't work. I had a second spare PSU cord and plugged it in and it worked. My other two cords that didn't work did work. Now after another shut down, my computer would only boot for a split second and none of my three PSU cords could remedy that. I don't have a 4th cord to test it out, heh. So I was wondering why just using different PSU cords fixed the issue of a split second boot and how can i fix that issue now without anymore different cords to use?