Have you checked the CPU socket pins on the motherboard?
Remove the CPU, use a magnifying glass to inspect the pins.
This link is for pins on a CPU but the theory it the same: How to Fix Bent Pins on a CPU
At this point you will need to swap out components in order to find the cause, either known good components in your system or your components in a known good system.
I would start with the GPU, then RAM, monitor, keyboard and mouse, (seems your PSU 'should' be good) even your PSU.
Yes, the keyboard and mouse can cause this error.
If everything is good then it's likely your motherboard.