Video quality glitches with slight rotation

I create guitar lesson videos. Whenever I need to do a slight rotation of the guitar video clip, the video looks weird. The strings get these intermittent dark patches on them, making it look like there is something wrong with the strings. As soon as I restore the rotation value back to zero it looks fine again. It also renders with this glitch so it's not a preview issue. Any thoughts on the possible cause of this would be greatly appreciated!