You said you had tested the PSU - Did you actually swap it out and try it on another system or use spare PSU on your GA-EP45-UD3R? If not, do that (It doesn't have to be a Gucci one - just any compatible PSU - so that you can test the Mobo's actually receiving and distributing the power...
Could you have accidentally left the CLR_CMOS jumper covered? (or are the pins bent and touching) - This would cause the same symptom. Something else you could check is all the PSU Power and small case connections - to ensure that the connecting pins have not slipped back inside their...
Wierd! Have you tried re-seating the RAM? If you have already done that, then you could try checking the system BIOS config. Usually there's a setting that influences how the system reacts to a power outage - Something along the lines of "On Power Failure" and options to Shut down...