RE: How to convert an ATX power supply to rc use
The gray to black might be the problem. If memory serves me, the gray wire is used to supply power to LEDs that remain on while the main supply is on standby, if you have it grounded it might be dragging the supply down causing the low voltage output, although I don't understand why it wouldn't be causing the PS to shut down. Try disconnecting them and see what happens. Usually the only two wires that need to be connected together like that to latch the PS on are the green and a black wire. I usually use the gray wire as the + wire to drive an LED I add as an "on" indicator.