Help! Please!

I have a project near completion but I am noticing some "clipping" in certain scenes. I am refering to clipping as a horizontal line that comes across the entire screen about a third of the way from the top of the screen. It is not present in the video until about 15 minutes into the project. I have several different scenes from several avi's but the line seems to come and go. Can any one help. This is important.

Thanks