Thin lines and sharp edges in images sometimes flicker when shown on interlaced displays, such as many TV screens. The Anti-flicker Filter control, located in the Effect Controls tab > Motion effect, can reduce or eliminate this flicker. As you increase its strength, more flicker is eliminated, but the image also becomes softer. You may need to set it relatively high for images with lots of sharp edges and high contrast.
You can change the intensity of the Anti-flicker Filter over the duration of a clip by setting Anti-flicker Filter keyframes at different values.