Constant shut down prompt if I don't use my mac every 5 seconds.

I know this may sound crazy since it works with Windows, but this only means that Windows probably isn't using or accessing the defective hardware in the same manner as macOS. I've recently found out through personal experience that macOS can do odd things when it encounters a hardware issue or it encounters hardware it is not expecting (such as using an Apple PCIe SSD from another slightly different Mac).

At this point you need to try to perform a clean install of macOS by first erasing the whole physical SSD before installing macOS. You can either boot into Internet Recovery Mode using Command + Option + R or by booting from a macOS USB installer. Here is an Apple article with instructions on how to erase the whole physical SSD using Disk Utility:

https://support.apple.com/en-us/HT208496#erasedisk

If a clean install has the same problem or the problem occurs while booted to the macOS installer (USB or Internet), then it confirms some sort of hardware issue with the laptop.

Does the same thing happen in Safe Mode?

Try running the Apple Diagnostics.

If the laptop was using macOS 10.12.6+ at some point, then try booting into Internet Recovery Mode (Command + Option + R) to see if the same thing happens.

If the same thing happens in Safe Mode & Internet Recovery Mode, then you likely have an issue with the keyboard or power button being stuck. You will need to have Apple or an Apple Authorized Service Provider examine the laptop to provide a repair estimate.

Edit: Sorry, replied to the wrong post.