Invisible lines on screen while gaming.

I am adding a new post to help explain the issue for you.

Most monitors run on a fixed refresh rate.

This means if your screen can display 60 frames per second and you push 70(or any number that does not line up perfectly, even a lower one) to it. some frames will switch to the next one before the image is fully displayed. This top to bottom scan seems to be a left over from the days of CRT's when images scanned from the top to bottom of the screen.

The reason Vsync fixes the issue is because the video card will ONLY send frames at the refresh of the monitor. Unfortunately this can cause some input delay while the card waits for the monitor to be ready for the next frame.
You can test to see how it works for you. It also works different with different games.

This can also cause half frame rates at some times without other settings(not too common). This is done to keep the frames in sync with the screen again(40 fps can not work evenly at 60hz so they drop to 30). An option called triple buffer removes this issue, but increases input delay further.

Please note that vsync and frame rate caps do not work the same. I can cap a game at 60fps on a 60hz screen and still get page tearing because the video card frames are not matched to the screen.

A newer option to avoid most of these issues is variable refresh rate. With variable refresh rate the monitor follows the video card instead of the video card following the monitor. In this kind of system the monitor refresh in constantly changed to match the video cards output. This means only full frames are shown and even wild fluctuations in frame rates do not cause this issue.

Variable refresh rate is currently sold under g-sync from Nvidia(only on G-sync screens with supported Nvidia video cards.). AMD has its own variable refresh rate using adaptive sync(a VESA standard recently added part of the displayport 1.2a) called freesync(a jab at nvidia's special hardware added to monitors for g-sync). Both offer the same features(Nvidia has an edge at this time because they make the chip used in all g-sync screens), but go about it slightly different ways.

I hope this gives you and idea of what it was and why you see it.