A problem with PC power supply

Hi all,
I have a so weird problem with my reprap Prusa (with RAMPS 1.4 circuit). If I don't supply for heating (conect the conector 12V, 11A) , everything is okie, the problem is if i do that the PC supply stops working immediately. I don't think the power supply has problem because I try replacing it with the same one, that leads to the same problem. But when I use 2 suppies for 2 conectors 12V, everything is okie again ^_^.
Anybody have the same problem or have any solution for that!
Thank you very much!

Check for shorts. Also you need a good load on 5V to get full power out of the 12V line.

As bryanandaimee said, I would re-check possibles shorts, or a electronic problem with the heatedbed wires.

You need at least a 300W power supply, twist half of the yellows and an equal number of black wires together and solder them. Do the same for the other half. You should end up with 2 large yellows and 2 large blacks. These are the input wires for the RAMPS 1.4 board. (If you have an odd number of yellow wires, give the extra one to the 11A input) (you may want to save one molex connector for a fan)

I've had success with this without having to load the 5V rail, but that may help as well.