Hi, There are quite a few threads about stuttering HD mkv's. I don't know it this can help in all cases but it did in mine. Problem free HD enjoyment really depends very much on the bitrate of the mkv. Many mkv's will play nicely in any player, but if the bitrate goes up to 20Mbit/s, all players struggle. My iMac with nVidia 8800+3.06Ghz dual core can not handle those with any player. My MBpro5,5 has an even harder time. The best experiences I have are with plex and Enhanced mplayer (enable multi threads). But even they struggle in critical scenes. BTW, this also applies to any windows PC unless the GPU supports the CPU. The CPU alone just can't handle it. Luckily for 10.6 users + 9400M chipsets and newer, apple included GPU acceleration for QuicktimeX. Unluckily, QuicktimeX does not use GPU acceleration for mkv's (actually, Quicktime does not support mkv's at all unless you install perian). If you get the GPU to support the CPU, the CPU load drops from maxed out to below 15% for exactly the same video stream. Here is how to do it: 1. Download avidemux2 (freeware, 2.5.2 is the current version). I think you will need perian (A52 codec) if you want AC3. 2. Open your video in avidemux2 3. set video and audio to copy (If you don't need AC3, I would use AAC since quicktime supports it better) 4. set the format to mp4 5. Save the video: if you got AC3 in the mkv, a warning appears that result might be unplayable. AC3 has only been added to mp4 standard recently, normally you have to use AAC. Ignore warning. 6. Video/Audio will be put into mp4 container. This takes a few minutes. No transcoding happens, only copying (passthrough). Therefore, quality will not be affected, output video will be the same size as original. 7. If you got AC3 in the stream, quicktimeX will not be able to open the video. The problem here I think is with the A52 codec in perian. You need to open QuicktimeX in 32 bit. 8. Enjoy video stutter free at 15% CPU. BTW: this only works for h264 coded mkv's which I guess are most. Cheers, Herbert. P.S.: You can do all of this also in Quicktime7 pro if you got it. Export the video to mp4, set video passthrough and audio to AAC. You will loose surround in this case however.