I have a Pan and Cropped video track on top of another video track that is fuzzy so I get a nice wide border effect for the main window - but I have a dark visable line on the running all along the entire upper edge of the Panned track - and it flutters too!
Could it be the Motion Stabilize artifact from your camera? If your camera has a Steadhihand type feature, the top of the frame will flutter as it tries to stabilize the picture.
Good question/guess?
But alas, I wouldn't know if it's "on" or not - new camera etc - but I'll check - BTW, can't I crop out the frame more to avoid this problem?