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What could cause intermittent boot failures in multiple PCs connected to the same power outlet?

 
I have a spare PC that has been running smoothly for over five years, but this year it started to exhibit a strange problem. Sometimes, when I try to turn it on, it does not boot properly - the lights and fans are on, but there is no display output, and it restarts itself after about 20 seconds. There are no error beeps from the motherboard. I have tried swapping the GPU, monitor, and RAM, but nothing works. I have also tested another PC with the same monitor and power outlet, and it had the same issue, even though it has completely different hardware.

 
I asked a friend who is a PC technician to help me, and he brought his own PC to test. Surprisingly, his PC also failed to boot in the same way. We checked the voltage and other parameters of the power outlet, and they seemed normal. After several hours of trying different things, the PCs suddenly started to work again, and they have been working fine ever since, until today, when the problem reappeared.

 
I am baffled by this situation, and I wonder if anyone has any idea what could be causing it. Is there something wrong with the power supply, the motherboard, or the BIOS? Is there some kind of interference or surge that affects the PCs? Is there a ghost in my house?

 
Please help me understand this mystery. Thank you.
 
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