Just sorted out my constant screen blanks on my nvidia 970 card.
What was happening was that occasionally one or the other, or sometimes both of my displayport connected monitors would drop out to a black screen. No "No Signal" or energy saving mode or anything appeared, just a black screen.
Anyway, I had a cheap bluetooth speaker on my desk and noticed that it happened as my phone dropped down to GSM and did its interference thing. Sure enough, make a call in GSM mode or send a SMS and one or the other screen will blank out. Its much worse if I have it charging on a USB cable that is bundled with the display port cables, but just holding the phone within 5-6cm of the cable while active will cause it.
Never happens with displayport off the onboard graphics.
Now I think about it, I recall the hassles I had with a phone and my HD60 capture box off the PC and the phone. I was assuming that it was the capture box being affected by the phone, but it may have been the graphics card causing dropouts etc.
In any case. If your screen is blanking out then move the 2g phone away from the PC. Or use a network that isnt so broken that you keep handing down to 2G.