Get a different PSU. That's an extremely terrible unit and certainly not reliable or of high enough quality to expect it to meet it's rated capacity. I'd never use a Xilence power supply with any unit, much less one with a high end graphics card or any kind of overclocking. You want a tier 1 or 2 unit of 500w or more as listed here:
http://www.tomshardware.co.uk/forum/id-2547993/psu-tier-list.html
Your unit is a tier 4 unit. It's not a "replace immediately" unit, but it might as well be with your configuration. I'd get it out of my machine before it damages something.