So I got up this morning and went to turn the PC on and the thing just kept switching off and on. Like it was trying to power on and then gave up half way through but a second later would give it another go. It just kept doing that until I switched it off at the PSU. It didn't get to post or beep, the fans barely started spinning before it would turn itself off again.

Thinking I've either got a dead PSU or worse, Mobo I was preparing myself for a day of diagnostic hell. I unplugged everything except the monitor and I didn't hold out much hope but I tried powering it on without and peripherals and bam, it powered up like nothing had happened.

I went back and plugged everything back in one by one, leaving out only a game controller I was no longer using and it's still working fine. (I may be using different USB ports now though, not sure)

It's got me stumped as to what the original problem was but due to the nature of the fault I'm still a bit worried. Anyone come across this kind of problem before and know what the core issue is? I'm wondering if I should be buying a new PSU or just hope for the best.

Deadlyf this has happened to me before and it was due to a poorly seated motherboard power cable. I just re-seated the power cables to the mobo and it fixed it. Exact same problem you just described.

Mine does this if it crashes from too higher an overclock, it does some weird boot/reboot cycle after which I hit the power button, then next time it turns on it tells me it failed due to overclocking & brings me to the CMOS setup where I can change the values of my failure to OC.

Also my PC will turn itself on in the middle of the night if I hibernate/suspend it. I don't know why (wake on keyboard/mouse is disabled). I think windows might turn itself on to run defrag or something.

When I say damaged front USB port. It was pretty mangled. Pretty sure headphones where plugged into it and it got push down while plugged in..