Im using a 250W atx , but I had to put a 2 amp load on the 5V ( regulated) rail to to keep the voltage up on the 12v rail, while its under load. At 7 amps the 12v rail drops to about 11v. I tried one 25w power resistor first as the load ,but it got very,very hot ,even at just 10 watts. I ended up putting two 25w resitors in parallel in an enclosed alum. heat sink on the top outside of the case, and now its just warm to the touch.
It works well, and was fun and cheap to do , but if you are powering anything that requires a sudden inrush of current , the power supply will shut itself down. I think that is typical of this type of power supply.