Does it work if you power off the PSU for several seconds and then power it on again instead of unplugging it?
The fact that you need to unplug the PSU leads me to believe that it could be the issue. Your PSU is a decent unit, but even the best can have issues. Ideally you'd try another PSU (borrow one if need be) before replacing the motherboard, etc.
Yup, it still works fine after that.