I have a UBMP 2.4 15" and under my user I can't play back the 1080p trailers such as astroboy without stuttering. If I switch to the guest account they play fine. I have deleted any quicktime prefs I have deleted all ByHost prefs I have run applejack to remove corrupted prefs I have install and uninstalled perian I have uninstalled flip4mac I have renames .cshrc I have removed all Login Items I have removed all launch agents in my library I have removed InputManagers and I'm sure a few more that I've already forgotten about. Both users show the same decoder, but the guest plays smooth and without difficulty what my users stutters to keep up with playback. Neither seems that different in terms of CPU and neither is getting anywhere near 100% CPU. I'm going insane trying to figure out why this works for one user but not for the other.