I'm surprised I'm still subscribed to this thread, but I started it, and I somehow figured out the problem, so I'll say what I can.

First question: When you first import a fresh clip, turn on the preview window. Does it show black border in there?

I think it may have something to do with the project size; in your case, the video not being wide enough to fill the whole project's dimensions.

If this is the case, you can change the project's dimensions (before importing any clips, I'd suggest), in File>Properties. Change the dimensions to form-fit your clips, import them, and see if that works.

I'll think some more on it, but...I'm tired

Good luck. Tell me if it works.