I'm having the same problem, I built a atx psu modified to a lab psu and I thought I could use this PSU to power my stepper motor at 17V, where -12v +5V = 17v but somehow the power shut down. I think the current is not hight enough to supply the motors. However if you check this link out: http://www.thebackshed.com/cnc/RouterA2.asp, he uses 2 x AT psu to achieve 12V at 8amps, so I might be right about the Amperage value.
Here is a photo of my little project: please open Attachment.
If you compare the power supply unit sold online or any other store cnc related projects, the size of the power unit is big and heave able to supply 48v, 10Amps......where as ATX psu....mm......slightly small in size, but not sure about the power, I guess dependes on PSU type.