I've done everything that has been suggested and nothing has changed!!
I even built a new box from scratch with Vista Ultimate. I didn't bother installing SP1, in fact I didn't install anything other than the latest version of
DVDFab (5040) and I still get the same problems. The box used is identical to the one I have at home.
I stick the same movie I've been testing into an XP box and voila! Works no problem.
I'm running out of ideas. As far as hardware goes the new boxes that I am having the trouble with use SATA interface DVD drives. I know it's coming from left field but I'm wondering if DVDFab has a problem with that.