GA-A75-D3H + A8-3870K Black Screen Problems - please help

[Swanny1]

Running windows 7 - keep getting black screen when using the PC and occasionally it forgets the front and or rear USBs. The MB, CPU and Ram have all been back to SCAN who say that they are not faulty. However, when I installed an external graphics card the black screen issue went away. Last night spent hours cleaning everything out - fresh reload of all software and appeared to be ok. Took it home plugged it in and same thing happened again. Any assistance or guidance would really be appreciated. I am sick of the thing. I have the parts since Oct 2012 and used it for approximately a couple of hours in total. Ahhhhh. Cheers. <o:p></o:p>