Cleaning the MAF sensor fixed my clutch judder. I have a 2005 IS250 with 130k miles that has juddered when cold for the last two years. Seeing this idea on another forum, I gave it a try and it's like a new car. No more judder, and much smoother and more powerful. I don't know why this works but a dirty sensor will presumably result in an incorrect fuel/air mix and poor running.