Try putting something on the 5v line like a fan or an old floppy disk drive.
I've got a Thermaltake PSU that will keep shutting down unless I do this and it's a well know issue when running without a motherboard!

I have a Seasonic 1250 running a Monarch with a fan attached to the 5V connector. But it worked fine before with only the 12v connectors attached and no fan.
But I'm also using this...
http://www.robotshop.com/en/cytron-atx-power-supply-breakout-board-right-angle.html
Although I don't think that would make a difference. It's just a fancy-schmancy paperclip, lol.