This has happened to me ever since I got a second monitor. At first, I only had a GTX 780, and I ran both monitors off of that. I recently acquired a GTX 1080. Main display runs off the 1080, second off the 780. The issue persists. I was told once that it may because the monitors have different refresh rates. I set both refresh rates to be 60Hz. The issue persists. No idea what could be the issue. Please halp me so I can actually make use of my second monitor!