I’m using the same driver you use (nvidia-legacy-390xx-driver) for my GTX 550ti, works perfectly, I have no tested mini HDMI, but both DVI work perfectly. I think the only difference (apart from Desktop GPU and Mobile GPU) is that I don’t like to use Hardware Acceleration on browsers (Firefox and/or Chromiun) so I have Hardware Acceleration (or GPU acceleration) Off in my browsers. As that models are no longer handled by nvidia maybe with browsers hardware acceleration on using the GPU comes this flickering issues. I will check tomorrow and update this post. Regards!