Display Flickering(After Motherboard changed and PSU repaired)

Hey guys i need ur help.. I am facing a problem and i have surfed thru entire net to find d solution bt couldnt , i guess u ppl can help me.
My motherboard is ga-78lmt-s2pt
Ram is kingston kvr1333d3n9/4g (2nos)
PSU-550 W(3yrs old)
Initially i faced a prob that when i started PC i showed blank screen.
Then i raised the voltage of ram to 1.66 from 1.5 v. Now the display is working. Bt when i connect any usb device(mobile , HDD) it start flickering and at times display goes blank even though processor fan is running.
Pls help