This normally happened to me when I overclocked my previous GTX 970 too high, with not enough voltage. If its not an overclock issue, it might be insufficient power to the GPU from your PSU, or the PSU failing. If its not that, it could be a few cold-joint solder balls on the BGA of your GPU motherboard. One way to fix cold joints is to take the heatsink portion of the card off and use a heatgun on it. If you don't want to do that and you've ran out of options, you'll probably be better off upgrading to a newer card anyway, such as the GTX 970 or 980 Ti.
Also, make sure the card is securely inserted into the motherboard. Sometimes they might get loose from fan vibration or heat - another bending factor that can also weaken solder joints.