I still like my theory, that Vader was just not "master" material like Yoda and Palpy. The physical and emotional damage he suffered after facing Obi-Wan greatly diminished his once-promising Force-potential, and Im sure that Palpy did not fully train him due to fear for his own life. I also recall reading one of the novels (I cant recall which, though it might have been Shadows of the Empire) where Vader is trying to breathe without his helmet. He removes it, and attempts to concentrate and use the Force to help him breathe. However, he is unable to, because he needs to remain angry in order to use the Force, and he cannot stay angry for long. I believe he was thinking about his confrontation with Obi-Wan, and blamed him for his state, but couldn't keep the anger long enough. I know this doesn't contribute much to this thread, but I thought it was an interesting piece of info none the less.