I described this problem some time ago along with the fix here: http://blog.brocktice.com/2011/05/10/too-much-power/

The problem is you need high-current-rated molex sockets on the ATX 12v pins. It's a cheap and easy fix. Even high-end PSUs like the Corsair AX1200 do not have these pins, an issue I bugged their tech support about.

H/T ArtForz for telling me about this in IRC ages ago.