200W is very low for a GTX 760. http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-760/specifications
The GPU alone takes 170W so that leaves barely any "space" for CPU, RAM and everything else.
I would recommend getting a 500W minimum depending on your CPU (E.G. if it's and AMD FX- then you'll need slightly more watts that if it was an Intel i7)