Strange Power issue

[vinegar]

I have a very similar issue. Built the PC and worked fine for years. One day it didn't power on at all when pushing power button. Tried again and it booted up fine. I happened to leave the PC running overnight. In the morning I shut it down. Since then it does nothing when pushing the power button. Now listen to this - I know it makes no sense. I bought a new PSU - same thing. I cut the wire from the switch to the mobo and connected it to no avail. So I set the wires back to use the switch and then I take out another motherboard I bought but never used (it was a replacement sent to me for a defective one I had bought prior to buying the one I am using) and tried that - same result. I bought a new CPU and heatsink, removed all peripherals - same result. I returned the PSU, CPU and heatsink and bought a new case (with a new switch) - same result. I left everything dismantled and powerless for a few days. Then one day I (insanely) tried again and when I connected the original PSU to the original mobo the PSU fan started going! For some reason it was on without pushing the switch at all. It went on and off with the PSU's built-in switch. So I rebuilt the whole thing back to the way it was originally and it booted up perfectly - I even logged in to Windows XP. Then I shut down gracefully - and it never came up again. Crazy. Any ideas???