Question Please help with first build, PC shuts off after 10ish minutes – even after RMA of CPU and mobo

Jun 22, 2020
Here's my part list -

So about a month ago I built my first ever PC build. I got everything together, tried to power it on, and things seemed to be going just fine. I decided to start to install Windows, but then after about 10-15 minutes into the process, it immediately shut down and would not start back up at all. The only way to get the PC to do anything more than a half-second flicker is to remove the power supply from the CPU (8 pin). I spent the entire weekend troubleshooting and trying everything I possible could: tried outside of the case, tried with/without RAM and one at a time, reset the CMOS, reinstalled heatsink + thermal paste, etc, etc.

I eventually gave up and sent the CPU and mobo back for an RMA repair thinking I somehow fried one of them in the initial installation or the seemingly 5 times I completely stripped this thing apart and rebuilt to try.

I just got my CPU and mobo back, and went through the installation process again, very, very cautiously this time. I got it all together, booted up briefly w/o a monitor to test, and things were going fine. I plugged it into a monitor and decided to try let it sit on the initial screen that loads when the PC posts, just to see what it would do instead of jumping into BIOS or Windows installation.

After about 5-10 minutes it did the same exact thing, and now I am in the same position I was in a month ago, despite having verified working CPU and mobo. The PC will not boot up at all, unless the CPU power is removed from the mobo. It seems like the RMA of the parts did something – because after the first time the PC abruptly shut off, it would not boot back up despite stripping it down and rebuilding multiple times.

My only thoughts are maybe it's the PSU or too much thermal paste. I realize I was adding more than required, but it still wasn't slathered on or anything. And if the PSU was the case, why would it be working for 10ish minutes?

Thank you so much for the help, this is my first PC build and now after failure a second time, I am beyond frustrated.