The more he tinkers with it, the more it exposes the flaws. I've always believed that SW was a great film primarily because of the masterful editing. The more changes, the more it upsets the original editing that turned a mish-mash of potentially corny scenes into a perfectly paced masterpiece.
I remember reading somewhere that the editors chose to edit based on the performances rather than on the original planned timings. That may be why the performances seem so good in the original, and get gradually worse as Lucas re-edits around his ideas, rather than around the performances.