[SOLVED] strange boot behaviour

A little stumped ATM.
The system runs just fine, when it boots.
Moe often than not it will not boot by just pressing the case button, it simply stays inert.
Flicking the PSU power switch has no effect but, if I remove and then reconnect the power lead, the system starts, just as if I pressed the case start button.
This is repeatable: NO boot from the case button, but if I remove and replace the power cord, even a few seconds after pressing the power button, the system boots as normal, even though I have not touched the case power button in the meantime..
So far all I've been able to do is the usual troubleshooting: Replace the PSU power cord and reseat all the MB power leads.

Right now a replacement PSU is en route and the current part is an Antec Edge 650, the rest of my specs are in my avatar.
This is really strange.
 

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