Maybe the card is getting tired. BGA solder connections deteriorate over time and with repeated heating, cooling cycles. Do a net search about baking a graphics card in your oven to reflow solder joints. It can work if this is the problem. I'd only try that as a last resort before actually buying a new one.
i5 3570k, ASRock z77 Extreme 4, 8 Gb Ripjaws 1600, Antec 430w psu, HVR 1600 tv tuner, custom case- marble top, oak face. Carver & DCM Time Window sound system. Mint 14 KDE.