I received my brandnew 2.8GHz iMac with a NVIDIA 8800 graphics card and 4GB RAM. Very speedy, but one thing bugs me: the 1080p trailers from the Apple website don't always play smoothly. There are some moments where the framerate drops down, which I think shouldn't happen on such a powerful machine. I also noticed it when watching 720p TV rips (.mkv files from Bittorrent) in QuickTime, which played smoothly in VLC though. There are no other programs running, Spotlight isn't indexing, and it's reproducable. One example I just tried: "Journey To The Center Of The Earth" trailer 4 in 1080p. At various times there is a noticeable drop in framerate. I saved the trailer to the hard disk and played it with VLC *no problem. What is the problem here? Why can't QuickTime handle it?