Something I have found with using computer power supplies is that they do not hold up to their rated amperage...even good ones drop a LOT under load.

I tried 3 different enermax and 2 antec power supplies... all of which report nice stable voltages on the 12V rail, regardless of what I did with the computer....but when I hooked up 2 5A chargers, I could charge at 5A on one of them... but as soon as I add even a 3A charge (giving a total of 8A charge...probably 9-10A draw) the voltage would drop below the threshold of both chargers.

All of these PSUs were rated at 16A or higher on the 12V rail and I couldn't even get half that.

Now, I'm using a proper hobby grade PSU (Protec 20A) and I can get my good LRP charger going at 5.5A, and both of my cheap chargers going at 5A and the input voltage doesn't even waver....