What I'd do is take it back down to bare bones out on a table. Nothing but
Now see if it will boot into the BIOS/UEFI. If not, try a different PSU and/or RAM. Once you do get into the BIOS/UEFI reset it to factory defaults.
Now power down, hook up your drive with Windows on it and try the same, (exercise it, web, games, whatever), and leave it running overnight.
If it's fine the next morning, power down and reassemble everything back in the case. As you continue beyond the bare minimum though, only add one or two pieces of hardware at a time and then test to make sure what was added isn't causing problems. You get the idea ...
- motherboard (on a non-conductive surface)
- CPU (w/heatsink & fan)
- RAM
- PSU
- video (including monitor)
- keyboard and mouse
Now see if it will boot into the BIOS/UEFI. If not, try a different PSU and/or RAM. Once you do get into the BIOS/UEFI reset it to factory defaults.
Now power down, hook up your drive with Windows on it and try the same, (exercise it, web, games, whatever), and leave it running overnight.
If it's fine the next morning, power down and reassemble everything back in the case. As you continue beyond the bare minimum though, only add one or two pieces of hardware at a time and then test to make sure what was added isn't causing problems. You get the idea ...