Ok, got a UMBP 2.4ghz 15". I'm using as a test http://www.apple.com/trailers/summit/astroboy/hd/ 1080p version. I always get judder, opening the inspector command-i shows that it's not keeping up but activity monitor shows that it's only using 20-30% of the CPU. Here is where it gets freaky. If I leave quicktime open and switch to another account it plays back perfectly, keeps up no problems and uses 60-80% of the CPU! I have stripped the accounts down so there is little or nothing extra running in the background and the system has plenty of ram. I have installed and uninstalled perian multiple times without effect. I have the 9600 GPU enabled and I'm tearing my hear out because it's just so bizarre.