Actually I've been having this problem as well. When turning on or waking up the computer, it sometimes turns off, turns back on, turns off again, then turns on and behaves normally. I am using alot of overclocking stability features so I assumed it must be that, but thats probably not the case for you.
My guess for you would be faulty PSU or mobo.
are you using an asus motherboard?
thats quite common on them
my P8Z68-V PRO/GEN3 did it until the latest bios update