Check to make sure the heatsink/fan on the processor is seated correctly? If it's on loosely and not pressing upon the CPU, then it's quickly overheating and shutting down the system....
If it's not on there properly, it's like not having one at all.. The reason it may work with the 12v plug not plugged into the board may be because the processor isn't getting power..
Processor not getting power, then it's obviously not heating up and shutting down the system.... I'd recheck this just to be sure....
Otherwise, you'll have to trouble shoot.. Maybe you can take out the PSU and run it up to a local PC shot and have them test it for you..? Without certain things, there's really no way for you
trouble shoot..
I'd say it's either the heatsink/fan not seated properly or it's your power-supply.. After reading many of the customer reviews at NewEgg, it's possible the board may be bad out of the box, but
not likely.... Same with the RAM... Have you used this PSU in a another computer and had it working??