Try looking through the help system. I believe that ithad some tips on when to use "quicktime margins" in titles, and when not to. I believe that using them incorrectly results in the kind of titles you've talked about, but then again it could be any number of things.
Note: I've witnessed less than optimal titles after messing with settings for very long periods. Perhaps iMovie can only render that well? The previews and sometimes the actual clips shown to you are merely superimpositions of standard text, and do not represent what the rendered text will actually look like.