Hi Grimy,
Welcome to the forum! Do you happen to have a spare power supply? I think that's the easiest way to confirm if this is an issue with the power supply or the motherboard. If you don't have a spare power supply you can also use the paperclip trick to test the power supply outside of your case.
There's a pretty informative guide below that you can consider following:
http://www.wikihow.com/Check-a-Power-SupplyOf course since there isn't really any load when you're turning on the powersupply through the paperclip trick, however a power supply that has 5V standby issues may have similar issue as what you described. So try using turning the power supply on/off a few times using the paperclip to see if the power supply can power on everytime. If after a few tries and it doesn't power on anymore then it's likely a defective power supply.
If you own a multimeter, then it would be even easier to check the voltages through the multimeter to see if your power supply is working normally.
Also try removing all your RAM and leave just one stick (test one stick at a time), it could possibly be defective memory that's preventing your system from tuning on and nothing to do with your motherboard or PSU.
Cheers,
Bahz
