Computer powers on, blank screen, shuts down after 30 sec

I just recently replaced a ton of parts in my PC after the BIOS on my old motherboard got corrupted. After the first motherboard went, I bought a new one. Which worked at first, because I got the BIOS at the very least. I tried reinstalling windows, and then my PC restarted mid installation and then I got no boot up every time I tried to turn it on after that. I decided to replace my motherboard again, along with my processor, my hard drives, and my RAM. I got everything fastened and in place, but now when I turn it on, I get no boot up, and after about 30 seconds, it shuts down, and if I leave it alone it will power on again by itself and repeat the shut down.

I am close to clueless about what's going on, I really hope it's not my PSU because it's basically the only thing I haven't replaced. Any ideas on what could be happening?

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Did you test it outside the case?

And no HDD either. We want to know it's not the case mounting issues or the OS.

Example at video:

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I'll try that

No I haven't tried that, I'll give it a go and come back with the result

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