It could be moire from illegal colors, it could be images are at too high a resolution for the render to manage, could be that you are seeing normal interlace artifacts on a computer monitor that won't show up on a standard television.
There are lots of threads on this subject if trying these things don't help.
I'd recommend reducing the size/resolution of a couple stills and replacing the originals on the timeline, and rendering those out.
Also, render at Best if you have a lot of stills/mostly stills.
Try inserting the Broadcast Filter set to Lenient.
You could try desaturating the images with the HSL, only slightly reducing saturation.
You could try dropping on a Gaussian Blur at .001 or .002.