If its turning on, then off - Usually there is something shorting it out, like dust behind the mainboard etc. Pain in the ass issues as they are hard to track down.

Best thing you can do is rip everything out, put it back piece by piece, and powe it up with absolutely bare bones. Eg: PSU, CPU, RAM .. and see when it fails to identify the culprit.

If it aint powering up with the absolute bare minimun, it's likely to be RAM (only put one stick in at a time if necessary), CPU or the board itself. If you have a newish PSU in, it's unlikely to be that unless you are overburdening it or it's a piece of shit yum-cha brand.

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Should have bought an STI